We have just one, and contrary to what philomom said, we don't stress out about much of anything. One of the reasons we're probably stopping with one child is because I feel as though with more children we would end up feeling pulled in too many directions (financially, emotionally, etc.)
All 3 of us are really relaxed, laid-back people. And Dh and I feel pretty confident with our choices for her. I feel as though if I had another child I would end up rethinking things too much, or worrying unnecessarily about the stuff that doesn't matter that much.
If anyone's interested in the environmental / zpg aspect, there's a book called "Maybe One" which is pretty good. Kind of dry and full of statistics, but interesting.