Three. Three is too many. Two is an ideal number, for if every couple were to have two, the population would remain stable. However, I realize that some people want to have multiple children for whatever reason, so here is what I would do about the situation. As Supreme Ruler of the American Empire, I would issue one "child-ticket" per person. This ticket would allow that person to have one child of their own (through egg-donors/baby mamas, if they don't want to get married or something). So when two people marry, they can have two children. However, if a couple only wants to have one child, or no children at all, they can sell their "child-tickets" to the highest bidder, so some other couple can buy up as many child-tickets as they please. You want to have twenty children? You're an idiot, but that's fine, get out your wallet.