Yep, that’s the other thing you have to factor in - how many pregnancies the average woman’s body can take before it’s no longer capable of sustaining them. Sure, some outliers can handle 19, but there are also going to be the outliers on the other side who sustain damage the first or second or fifth time that makes further pregnancies impossible.
Also, around 20-30% of births in developed countries are by C-section. In the US, it’s one in three. After two or three of those, you shouldn’t aim for a vaginal delivery because of the risk of uterine rupture, so you’re stuck with more sections - and after enough (RivkahChaya mentioned four, which is what I remember too), your risk of rupture goes so high that pregnancy just isn’t safe. And the rate of complications necessitating sections goes up with age. So if this average woman has two sections in her first ten births, she’s on borrowed baby-time after that. If she’s in the US and has three sections in those first ten pregnancies, then she’s down to one more pregnancy.
Breast-feeding, by itself, acts as birth control, though, on average suppressing ovulation until around 8-9 months after birth, and usually longer in cultures where babies are exclusively breastfed for more than six months. For a woman with 8 kids, that’s another six years or more during her most fertile years when she won’t get pregnant. Further, having “lots of help available” isn’t quite the same thing as “having no responsibility at all.” Orthodox culture is closer to the situation I asked about, but still isn’t quite the same.
Yes, c-sections do act as a constraint. However, most researchers seem to think that the American rate (one in three) is way higher than is medically appropriate; the World Health Organization suggests that rates of 10-15 per 100 births yield the best outcomes, and that increasing c-section rates above that level are associated with HIGHER maternal and infant mortality. (cite) That suggests that an average woman with ten kids should have had one or maybe two c-sections, not 3 or 4.