I think there shouldn't be a limit per say. But there should be a limit according to wealth. If you're poor as dirt and can't even take care of yourself, you won't be able to take care of a child, period.
Maybe there should be a limit actually. I don't know, 3 per person? That sounds ok. Not 3 per couple, 3 per person in their lifetime. Don't know what's to be done in case someone makes twins or more at one birth though... 3 successful births per person? Meh, this is complicated, then people would start getting special fertility tretments since they give higher chance of more children...
Ok, 3 successful children per person (this means a woman can have 3 children, but the man also 3, they can be shared, as in the 3 belong to both the woman and the man, or the woman can have one with another man and 2 with her husband and the husband have 1 with another woman, hope you understand the principle).
In case of twins, triplets etc... ummm... I don't know, I honestly have no clue. They could be counted as 1 I guess.