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.