According to the Guinness Book of World Records, the 18th century Moroccan ruler Ismail the Bloodthirsty
holds the record for being the most prolific father ever. He supposedly fathered 888 children, which means he had to father about 15 children a year for 60 years.
But Dorothy Einon, a researcher at University College London, argues in her article "How many children can one man have?"
that even if Ismail had access to a steady supply of fertile women, it would have been impossible for him to father this many children.
The infrequency of ovulation. Ismail would need to accurately time when the women were actually fertilizable, which is a fairly small window of time each month.
Women who mate infrequently have longer cycles and ovulate less frequently. So if there's this huge supply of women mating exclusively with Ismail the bloodthirsty, then each woman is mating infrequently, and thus the odds of mating when she's fertile become even slimmer.
The low incidence of conception. Even if Ismail managed to copulate with a woman at the ideal time, and an egg was fertilized, only 42% of fertilized eggs survive to the 12th day of pregnancy.
The high frequency of infertile women, especially in the developing world, which reduces Ismail's odds even more.
Not all women are chaste. It's not logical to assume that all those women were mating exclusively with Ismail. Therefore, one can't assume he was the father of them all.
If Ismail copulated with multiple women every day, his own sperm count would drop, further reducing his chances of impregnating a woman.
Einon concludes that, given all these problems "unless a man has an extensive harem and a good harem keeper, it is unlikely that the extreme range of male and female reproductive success is very different."