[CalRobert]

At what number would you consider it a problem?

For what it's worth, I would be delighted to live on a planet of 10 billion people who have found a way to live in a way that can persist for millenia, but we don't seem to have done so. Although, living in a well-insulated flat made with renewably-grown timber (a great carbon sink!) walking and biking distance from all of my daily needs where my meagre energy needs are met with 100% renewable energy (when I lived in San Diego I worked from home and our apartment's average load was 100 Watts so this can be done!) on a mostly-vegan diet and working 20 hours per week or less sounds pretty nice to me.

[gsich]

Probably 200 billion, going by population sensity.