I wish I had a dollar for every time I rambled on about a much needed global population control, especially if some third-world countries with massive populations (like India and China) ever reach a first world status and everyone there is going to want their own house and car
2 is enough...globally. 2 in theory is enough to replace each parent when they die, so things stay balanced, especially the ratio of young people to old people
Two kids. A kid's gotta have someone to confide in, and that isn't always the parent. There's just some things kids don't want to tell their parents, but would willingly tell their sibling (I'm talking from experience). It might also make them less selfish if they have to share their parents' attention between the two of them.
1 ideally (for me) because there are already so many people, there's always adoption if you want more ._. I can see why someone would want more of their own, though :\ I say if you have the money to have tons of kids, then go for it. It's too much when you have 3-5 and you can barely put food on the table. Clothes, food, living expenses and schooling is expensive, especially since a lot of parents aren't able (and unfortunately, there are some that are unwilling) to seek better paying jobs.
If every person limited themselves to 2 total children the world population would stabilize and slowly over time decrease, however given that many parts of the world already suffer from massive overpopulation, 1 would be better.