I cant begin to imagine how hard it must be to raise a large family, but clearly lots of parents manage well (though personally I could not unless I could have a nanny, cleaner, tutor and enough money for a bigger house etc!)
I think it's just a case of effort X resources. I think the maximum sensible number of children would vary massively from family to family, but even with unlimited resources and 2 dedicated parents I imagine there's probably a limit beyond which no parent would have the time to be close to every child, for example, the TV mother with 18 children (15 at home) and 5 dogs who does not have a lot of money.