"The researchers, led by Ken Nakayama and Richard Russell at Harvard and Bradley Duchaine at University College London, have found evidence that prosopagnosia, once thought to be exceedingly rare, may affect up to 2 percent of the population – suggesting that millions of people may be face-blind."

http://news.harvard.edu/gazette/story/2006/06/face-blindness...

"Recently, a hereditary sub-type of congenital prosopagnosia with a very high prevalence rate of 2.5% has been identified."

http://onlinelibrary.wiley.com/doi/10.1348/174866407X231001/...

Presumably most cases are much milder.