Here’s how I think about it:

If a couple has two kids, they only replace themselves after they die. 
If they have three, there is a “profit” of one. Population wise, this would likely be canceled out by couples having no or one child, but it’s still growth in the context of that nuclear family. That’s my baseline goal as far as number of kids. 

I was an only child so my stretch goal is four or more if I can. I probably won’t be able to have more than three, though. I’m not too tore up about it; I’m more focused on enjoying and doing the best for whatever number of children I’m blessed with.