Below is how Asimov in 1964 came up with the guesstimate that "the advance of science throughout history has depended on the eagerly working minds of no more than one [human] in six million".
(You can drag the blue numbers to alter their values and the result will change accordingly.)
If the current world population is billion, and if that number makes up of percent of all human beings who have ever lived, then billion men and women have lived and died on Earth since our species arose.
If the number of all great scientists is , and suppose that the total number who have contributed significantly is times as great, which is , then
"the advance of science throughout history has depended on the eagerly working minds of no more than one [human] in million."
Notes: The default numbers above are data in 1964. For reference, as of March 2016, world population was estimated at 7.4 billion, and the Population Reference Bureau estimates that 6.5 percent of all people ever born are alive today, instead of 5 percent.