Louis used to cycle to the sperm bank, his deposit in a bag. He needed to make it in good time to preserve the contents of each specimen jar, which he placed in a warmed cabinet when he arrived. In the evenings, when men arrived after work, there would sometimes be social events at the sperm bank, with tea and cake. Other sperm banks – he was a regular donor at three – were more perfunctory, with small rooms for donations and the usual magazines.

Like most of the donors, Louis preferred not to linger, but to pedal back to his modest flat in northern Holland and a life he felt was so ordinary that it almost blurred into the background. He was in his early 30s and lived alone, working as a bank clerk. He had no girlfriend, nor any close friends or family.

But Louis was on a secret mission, motivated by a deep anxiety that had built as he drifted through early adulthood. Profound questions of mortality were keeping him awake at night. “I had started to think, ‘Who will remember me when I’m gone? Who will talk about me? Who will be my heir?’” he says. “I think our biggest fear in life is not to die, but to be forgotten.”

So Louis made an audacious plan. If he wasn’t going to have children of his own in the normal way, maybe he could donate sperm in such quantity that – eventually – a child might try to find him. To pull it off, Louis would need to play a biological numbers game. “If I had 10 children this way, there would be a very slim chance of success,” he says. “But what if I had 100… or even more?”

***

Talking publicly to the English press for the first time via a video call, Louis, now 68, wears a denim shirt and occasionally waves his arms at the screen, which is propped up on a pile of books. “Louis” is an alias he has asked to use because, among the other consequences of his actions, he has been threatened. Speaking deliberately, with the fluency of a man who has had a lot of explaining to do, he describes how his mission has transformed hundreds of lives, but also raised questions about family secrecy, identity and the ethics of artificial insemination.

Louis was born in the Netherlands but spent his earliest years in Suriname in South America, where his father, a doctor, was born. Louis barely saw him and he and his mother, a Dutch missionary nurse, returned to Holland when Louis was six. His father later also settled there.

Louis and his father were largely estranged, but the boy felt a pressure to seize opportunities his Surinamese family never had in the former Dutch colony. Even so, he struggled at school and dropped out of university. “When I was 21, I got a job at the bank and I sat behind a typewriter, which then became a computer, for 39 years,” he recalls. “My father was never able to understand that. He felt like he had given me a chance and I hadn’t taken it.”

A broken home turned Louis against the idea of marriage. Had his parents been happy, he wonders if he might now have siblings and closer ties to his extended family. Perhaps he would have a family of his own. He also believes he has a form of autism, which has made relationships and emotions difficult. He says he doesn’t feel things like other people. Romances have died on the vine. He was happy in his own company, but an existential angst still consumed him – until he decided on his unique mission.

I was pregnant with twins when I found I had 15 siblings and that my father was not my father

Regulation of Dutch sperm banks was lax in the early 80s, but Louis knew that donating at the level he needed would be discouraged. (Laws still vary. In Britain today, the same donor sperm may be used in no more than 10 families. In Holland, the limit is now 25. Any more, and the risk of accidental incest is thought too great.) To avoid raising suspicion, Louis used three sperm banks, the farthest one a short train ride away, logging his donations in a notebook he still keeps on a shelf.

For 20 years, from 1982, Louis donated as often as three times a week, generally before work. He says the banks must have known he was visiting too often, but demand for reliable donors was high: Louis was an asset. The clinic he cycled to, it would later transpire, had also exaggerated his credentials in the anonymous profile for prospective mothers. “It said I was university-educated, that I was a boss at a bank and that I had no interest in being contacted by future children,” he says. It also failed to mention his ethnicity; Louis describes his father as “black and white – we descend from African slaves and their owners”.

While the sperm banks turned a blind eye, Louis never lost sight of his goal. He describes it as a train that was hard to stop. Eventually, in 2002, he felt as if he had done enough. By then, Louis was in his 50s and his oldest children – wherever they were – would be adults. He returned to his quiet life and waited.

***

As she grew up, Joyce Curiere, now 34, became aware that she did not look much like her blond, blue-eyed father. She has freckles and thick brown, curly hair. “My parents would say things like, ‘Well, you look just like your grandad’s grandad,’” says the former nurse, who now works in sales in Amsterdam. When a grandparent let slip the truth – that her parents used donor sperm – Joyce, then 16, confronted them. They refused to admit it. She wanted to know more, but had no idea where to begin.

In Britain, donor children born since 2005 have the right to find out the identity of their biological parents when they reach 18; Holland has introduced a similar law. Children born before 2005 do not get the same right to know, but what has changed for them is the rise of consumer DNA testing. Services such as AncestryDNA and 23andMe offer low-cost reports in weeks, including genetic matches to other people who have submitted samples to their growing databases. In Holland, several family reunion TV shows have emerged in this era of genetic accessibility. In 2015, 14 years after she confronted her parents, Joyce sat down one night to watch Familie Gezocht (Family Search). It featured a woman called Amanda.

Jordy Willekens, who made Louis his legal father. Photograph: courtesy of Jordy Willekens

“She was a nurse, too, and had curly, brown hair and the same voice as me,” Joyce says. “They showed pictures of her as a baby and I was like, ‘Well, that’s me. Those are my pictures.’” Amanda even wore similar earrings, and had a tattoo on the same shoulder. Two weeks later, Joyce sent off a DNA sample to a Dutch foundation that helps unite donor families. “I was pregnant with twins when I found I had 15 siblings and that my father was not my father,” she says.

Those 15 people had already started their searches, and submitted samples. So had Louis. The children matched each other – and him – and the number quickly grew. Louis has calculated his donations have resulted in 200 births, “give or take a dozen”. He doubts another estimate, which he says came from the foundation, that he may have as many as 1,000 children. Confidentiality and use of unlicensed donors means records are hard to keep, but a figure anywhere near this high would make Louis one of the world’s most prolific fathers.

For Joyce, the revelation was overwhelming. Yet her parents refused to accept the DNA evidence. “Their doctor said never to tell their child she’s from a donor because it would get them in trouble,” Joyce says. She is now estranged from them. After a difficult pregnancy, followed by postnatal depression, she did not have the time or emotional strength to meet her biological father. Not yet.

***

Four years before Joyce’s discovery, Louis happened to see a TV show called Wie Is Mijn Vader? (Who Is My Father?). “They had a couple of twins and a sister, and at first I didn’t react,” he says. “Then they said the names of the boys who wanted to meet their father. They were Maaike and Matthijs. I thought, ‘Shit, those two are mine! It is really happening.’”

These names had meant something to Louis since the autumn of 1989, when he had spotted a document he should not have seen at one of the sperm banks. It had the names of two children – Maaike and Matthijs – and some basic information about their donor: dark hair, dark eyes, A+ blood type. “I thought, that’s me, those must be my children,” Louis says. He made a note of the names. “I call it divine intervention that made me look at the right place at the right time,” he adds. “Just as it was in 2011, when I watched that TV programme.”

Ivo worries about his children having romantic meetings with relatives. ‘What if they party in the same city centre?’

On the show, the twins talked about hoping to meet their father one day. “I looked more closely, and they looked like me,” Louis says. “I don’t feel feelings, but I can say that this was like an explosion.” He contacted the TV company, and submitted a DNA sample. He got a call to say eight children were, at that stage, looking for the same father. The first meeting came later in 2011, when Louis arranged, through the foundation, to see a brother and sister. “There was no crying, but they did find it very intense,” Louis says. “It was difficult, because I thought they would want to know everything about me, so I gave them everything, and it was a lot. I had to learn to listen. But they were mine. They really existed.”

In the seven years since, Louis has met more than 40 of the 57 children to whom he has now been matched. Joyce emails me a list of their dates of birth and initials. Joyce is among the oldest, and was born in March 1984. The frequency of the dates suggests a village school, rather than a family. Children known as I, B, M, K, H and G were also born in 1984. H and J arrive in 1985, and six more in 1986.

A few months after her own twins were born, Joyce decided to meet up with Louis. She remembers being struck by how alike they looked. Louis talked and talked. “All the things I had wanted to know about myself – all the answers I had been searching for since I was 16 – he gave me in 90 minutes,” she says, sitting next to Louis (he has travelled to her home for the interview). He had a dark sense of humour she immediately recognised as her own: “Sometimes I still sit with a cup of tea and ask, ‘How did this happen?’” she says.

Watching a TV documentary featuring some of Louis’ children, Ivo van Halen realised he must be one of them. Photograph: courtesy of Ivo van Halen

She also got to meet Amanda, the sister with the same tattoo she’d seen on TV. All the siblings describe an instant, reassuring recognition when they first met – an immediate ease; a shared sense of humour. This is captured in a striking clip from a documentary they contributed to in 2015. When Joyce answers her door to Amanda, they are wearing very similar floral dresses. They have the same stature, hair, eyes. They head to the park and toast each other with a glass of wine. “To being sisters and finding each other because the universe wants it that way,” Joyce says. “It’s as if we’ve known each other for years.”

***

With each documentary and news report about Louis’ children, awareness of his story has grown. So have the Facebook and WhatsApp groups the siblings use to communicate. The web got more tangled still when one mother was angered to learn her donor was mixed race. She talked to a Belgian newspaper and, Joyce says, a neo-Nazi group threatened to “come for Louis”. Some siblings were against even an anonymous interview, partly out of concern for his safety: “But I think he should tell his story now,” Joyce says, “and not just be talked about.”

Louis is matter-of-fact about the ethics of his endeavour. He feels no remorse, but for some the experience has been fraught. Each child arrives with their own story – and questions. Ivo van Halen, 34, learned only recently that his parents had been preparing to tell him the truth about his parentage, until his mother died in a car crash. Ivo was 11, and his father lost the courage; he couldn’t risk losing his sons. They had his hair colour and never suspected he was not their father (Ivo’s brother is from another donor). But five years ago, Ivo’s father called his sons together and told them. “We said it changed nothing,” says Ivo, who works in IT. “I felt sad for him, that he’d had so much difficulty with it.”

Ivo felt little need to find his biological father until he, too, saw a documentary featuring some of Louis’ children. The story had now become part of a bigger scandal in Holland, after it was revealed that the doctor who ran one sperm bank Louis used had secretly donated his own sperm, and mixed it with other donations to increase the chances of conception (the doctor has since died). Watching it, Ivo says, “I thought, ‘Wow, these people even move like me. OK, now I want to know.’” He did a DNA test, and joined the Facebook group.

Ivo has not yet met Louis, though they have exchanged emails. He struggles to explain the delay; he appreciates knowing the truth about his identity, and has enjoyed meeting his siblings, but not for the same reasons as Louis: he has young children now, and wants to protect the next generation from the risk of romantic meetings with distant relatives. “What if they all go and party in the same city centre?” he asks. “I want to prevent accidents from happening.”

Joyce is one of the three children to whom Louis has grown closest. Photograph: Judith Jockel/The Guardian

“There are bigger threats in this world,” Louis says in an email when I press him on these risks. “But if Ivo’s concerns bring new sheep to the flock, who am I to protest?” There is some ego in his mission. He is only half joking, during the video call, when he reaches to ancient Egypt for a metaphor. “The pharaohs built pyramids,” he says. “These children are my pyramids.”

Louis, whose own parents died in 1988 and 1990, is close to only a handful of the children he has discovered so far. On the day I talk to Ivo, the group of siblings receives a message from Jordy Willekens, who is 28 and has been a member for three years. He has news to share: he has decided to make Louis his legal father. Jordy always knew he had a father out there, because he grew up with two mothers. He thinks their honesty partly explains why he felt little need to find his father. He also assumed it would be too much effort. “If I had realised it would be this easy, I would have done it 10 years ago,” he says.

Three years ago, Jordy’s then girlfriend alerted him to a documentary about the group of siblings. “It was madness to think I could deny it,” he says of the physical likeness he saw in them. He got the test, joined the group and, in 2016, met Louis in a restaurant. “He stared at me for 10 minutes,” Jordy recalls. “But it wasn’t awkward. It felt like I had found half of me.”

Jordy, who also works in IT, grew fond of Louis. He gets him. “We’re all afraid eventually to turn up alone without anyone to love us,” he says. “I wouldn’t go to those lengths, but he never had anyone to tell him it wasn’t ethical or that he wasn’t alone.”

Six months ago, in the shower, he decided to make the legal change. The paperwork came through last week. As we talk, Jordy gets an email from Louis asking if he has shared the news with the group. “He has addressed it to ‘My dear son’,” Jordy says, laughing a little.

Louis says Joyce and Maaike, the other two children to whom he has grown closest, are also now in his will. He is not a rich man, but says he always hated the idea that strangers would one day come to clear out his apartment and arrange his funeral. “Now I have three heirs,” he says after Jordy’s news breaks. “Perhaps there will be others. Mission accomplished.”

If Jordy now sees Louis as a father, for some it’s not quite as clear. Joyce, who sees him only occasionally (they live in different cities), describes him more as a good friend she can phone for advice. She has lost contact with the father who raised her, but says she isn’t looking for a replacement.

Seventeen of the siblings are members of the WhatsApp group. They talk about work and share memes in the usual way. “You can discuss sperm only so much,” Jordy says. “But there’s always a buzz when new blood is added to the mix.” Not that everyone wants to be part of this strange new family. For some, the shock of discovery is too much. “When I met one daughter, it was as if all the energy in the room blew away,” Louis says. “I didn’t know what to say, she didn’t know what to say, and later she told me she didn’t want to meet again.”

Jordy’s biological mother, who separated from her partner years ago, had no idea who had helped her have a child, or why. She was initially concerned about her son’s search, but approved once he reassured her it could never change their relationship. Last summer, she and Jordy met Louis together. “I was walking behind them at one point and I was like, is this really happening?” Jordy recalls. He sees Louis as a real father now – just not a straightforward one. What did his mother think of Louis? “She liked him,” he says. “She said to him, ‘We created something really nice.’ And he agreed with her.”

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• This article was edited on 7 December 2018 to clarify that Louis was talking to the English press for the first time.