25 pairs of people originally, each couple miraculously giving birth to 4 children each. This makes 100 people by Generation 2.
Totally assuming that there would be no infertile people ever, and assuming that incest is ok (since that would dwindle the numbers of compatible non-incestual couplings SIGNIFICANTLY by generation 3 or 4), and assuming everyone has children at roughly the same age at roughly the same time in their generation, and assuming that none of these kids die, and assuming every person dies at the age of, say, 60 ...
bleh I don't have time for these numbers!!!
Anyway all it'd be is a lot of useless and impossible assumptions, paired with exponents, while subtracting the dead. And the numbers REALLY wouldn't explode that much, as mortality strikes anyone at any moment, and lots of people aren't that fertile, etc. etc.
It's a useless discussion really