I haven't a clue what a sustainable number of people would be. I know that it is way, way less than we've got. Particularly as I feel there is value in having other species living on this planet. Wild ones, not just the ones that have been harnessed for companionship, food, or labour. Re-creating enough habitat for other species to thrive will take lots of time and a huge reduction in our numbers. Sorry I can't define huge in this case--I'm a geologist--it would take a biologist to do the calculations necessary to determine how much land and what types are needed to bring us a planet with plenty of biodiversity. I have no idea what sort of scientist it would take to then calculate the number of humans that would be "right" to provide an "acceptable" level of technology without doing damage to the planet. Probably would take a seriously inter-disciplinary project.