Guessing the number of billionaires in the world (or even in a single country) is just that — a guessing game. But it's a highly profitable and increasingly popular one.

Bloomberg does it. Wealth-X does it. Forbes has been doing it for more than 30 years.

The latest guesstimate is from The Hurun Report in China. Hurun said there are 1,453 billionaires (in U.S. dollars) worldwide. They had a combined fortune of $5.5 trillion — equal to China's gross domestic product.

The average age is 63. One in 10 is female.

The "10-zero-club"— individuals with $10 billion or more — now totals 108 billionaires. Topping the list is the same trio that tops the other billionaire lists: Carlos Slim, followed by Warren Buffett, Spanish fashion magnate Amancio Ortega, Bill Gates, and LVMH chief Bernard Arnault.

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The wealth of the Top 10 grew $250 million a day in 2012.

The Unites States has 408 billionaires and China, said Hurun, ranks second with 317

But Rupert Hoogewerf, chairman and chief researcher of The Hurun Report, said the 1,453 billionaires is probably an under-guesstimate. He said the real number is probably three times that.

"For every billionaire that Hurun Report has found, I estimate we have missed at least two," he said, "meaning that today there are probably 4,000 dollar billionaires in the world."

And why are those other 3,000 or so billionaires hard to find?

"Some people deliberately make their wealth a secret because ... they gained it through illegal ways," he told reporters at a press conference. "Some others simply prefer to keep a low profile."

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In 2011, Hoogewerfe said China had 189 billionaires "or maybe 400 or 500." So China's billionaire population has possibly doubled in two years — or maybe fallen by half.

Welcome to the billionaire guessing game.