Most theists (perhaps, most people in general) would say that having a happy person is better than having no person at all. Indeed, most theists admit to this by saying that God "wanted to" or "lovingly" created humans, which suggests it is a moral act.
So, it would seem that, altogether, having more people is better than having less people. If we assume that a theist admits > 50% of people go to heaven, it seems that having the maximal number of children is the best thing one can do in life. Even if one says that there are other goods that are given up when having so many children, it seems that an infinite good is given up (an eternity of happiness for a person).
However, most theists do not advocate having as many children as possible, especially at very young ages. How can this be justified?