When Tigran Sloyan, co-founder and chief executive of CodeFights, was looking for funding this year, he wanted to get a well-known angel to commit. He approached Adam D’Angelo, an early Facebook employee and co-founder of the question-and-answer site Quora.

“When I pitched the idea to Adam, he was super on board,” Mr. Sloyan said. By March, a month after Mr. D’Angelo agreed to invest, CodeFights had raised $2.5 million from more than 30 investors. “After they see a name they trust, everybody wants to invest,” Mr. Sloyan said.

In an email, Mr. D’Angelo, who began angel investing after cashing out of Facebook, said that he was drawn to the experience of CodeFights’ founders, the fact that consumers were already using the start-up’s products and the company’s potential market size if things go well. “There is a lot of potential here,” he wrote.

That’s in contrast to the handful of angels — usually five to 10 people — that start-ups typically had in the past. “The total angel scene back when I started in 2006 was 30 or 50 angels, maybe with three to five very active ones,” said Aydin Senkut, founder and managing director of Felicis Ventures, a Silicon Valley firm that focuses on early stage investments. “Now everybody and their family and their pets who have some money want to get into angel investing.”

The growing numbers of angels have spurred competition to get into deals, leading to some questionable practices. Spenser Skates, co-founder and chief executive of Amplitude, said some angels were so eager to put money into start-ups that they barely glanced at deal terms.

“They don’t do the same sort of diligence,” Mr. Skates said. “They hear your story and you send them the paperwork.”

Too many angels can also hurt a start-up and be taken as an indication that a company is not strong enough to attract any one “lead” investor, said Ms. Hua of Apptimize. Many angels may also not know the exact health of the start-up they have invested in when asked by others, which can give the perception that the company is not doing well.