I don't know. I have three. Four would be too many for me. I think most people stop when they feel a squeeze in resources, whether that's time, money or patience.
As for too many. I don't know. Enviromentally speaking, probably one. People will be along soon, if they have already as I am the slowest ever typer, and say that when you have X number of children that they don't get enough attention. I think that's flawed thinking tbh because it deminishes all the added attention that you get with having an abundance of siblings.
That being said, you couldn't pay me enough to have more and test my theory.