I get that there might be exceptions for real, there is no one rule that can ever work for all. However, I still think a family's income should be considered when it comes to deciding the number of kids they can have. It is a good thing for the family, for the child, and also for the country.
I do agree that some people actually needs to be educated about topics like this, I think I mentioned that somewhere in the post. I know of cases of when a family will not really plan for a child and it will come. Also, imagine a case when you are preparing for a child and two or three pops out. Lol.
The rule should be very flexible, if you can afford to give birth to this nnumber of children, go ahead then. Also, if the country is in a good state, it can help boost the income of the family in order to take care of themselves without any hardship.