The number of kids one wants to have should depend on one's decision and this decision should be based on the capacity of the person to be able to take care of the children. I don't think there should be general rules on how many number of children people should have, except in those country with explosive population. And due to this, the government might decide to place limit on the number of children a certain family should have. But when it comes to other countries that does not have such laws in place, their population can go ahead and give birth to the number of children the parents can cater for.
Sadly, it doesn't really work that way in many families, as some go ahead and give birth to quite large number of children they can't meet their needs. And as a result of this poverty is now order of the day in the lives of such children.
If I am asked for solution to the problem on how children needs are left unattended to materially, financially and most of all emotionally by their parents, I will say there should be law that would limit uncontrolled giving birth to children.