The main source for the estimates is a study made by a group of psychologists at Stanford University, California. Its origin is from 1926, but it has been revised a couple of times. The last time, is by Dr. Catherine Morris Cox in The Mind, Life Science Library Series, 1964.
The rest is from various sources: Encyclopedia Britannica, Guinness Book of Records, Popular Science (Swedish magazine), Daily News (Swedish newspaper), professors in Mathematics, a comparing study between the geniuses of their centuries in Mensa Research Journal, Buzan, etc.
The estimates are what they are - just estimates. They can’t of course be justified.