2? One to replace mom, one to replace dad? Or 1 like the Chinese to get the number of people down which after peak oil will be unsustainable?
It depends upon a number of things. Basically it boilds down to how many people do you think the world can support. Or, you could also consider how many people you think your local region can support. Some do feel that we have already passed that point, in which case the answer would be 1. Others feel technology will allow our globe to support many more people in which case the answere would be 'as many as you want'. Personally I am very happy playing uncle as I can spoil the kids and them give them back
Single most important rule of parenting: If you only have one, they can't fight in the back seat and you always know which one did it.
one must also factor in the likelihood that their children make a major breakthrough socially or scientifically to make the planet more green...
That's a nice piece of positive thinking there, but you would need to consider that right now there's a thread of green-minded people talking about Swedes burning wild bunnies. I'm pretty sure my parents never saw that one coming.
Create no more than you can properly care for, and in any case not more than two. But feel free to adopt as many as you can love.
Having one child will increase the population before it will decrease it. And if you have that one child early while you're young, and your child has one child early while they're young, then you are still increasing the population. So zero it is. But I like richard schumacher's answer best.
Or invent a vehicle larger and more wasteful than a Hummer, or write a major best seller "proving" pollution is a hoax.
