There is no such limit as I know of. There is a modifier that drastically reduces the chance of pregnancy for the mother every time she gives birth to a child but no hard limit in terms of children count.
Roughly speaking if the chance of having the 3rd child is 1, the chance of having the 4th child is 0.44. However do not forget as the wife ages, the chance also diminishes linearly by age, becoming 0 around ~40. If the chance of an 18-year-old wife becoming pregnant is 1. When she becomes 30 years old, this chance becomes 0.5.
Though you have to multiply the modifiers to arrive at the chance.
It is hard to have more than 3-4 children, but not impossible.