I don't have data to back it up, but I'm pretty sure the absolute max is 6 per female, but is random between 2-6. It seems if you want to maximize your kid output you should play as a male and find a non-combat oriented wife who is young (better fertility compared to older ones) and after she's had ~3 kids, which seems to be the average, have her killed then marry another wife (it's really difficult to kill them for some reason, though, even with chance at 100%). Rinse and repeat. I'm 99% sure male character's age doesn't matter for fertility.
When I play as a guy I do this, and train one kid and maybe a backup to be my heir while all the others I train in Trade so they can run caravans. It also helps if their wives have some Trade skill, then all of them can run caravans! Be sure not to marry off your daughters so they can stay as caravaneers