I think the general hope is that population growth will halt somewhere between 10 & 11 billion. I say hope, rather than projection because, while many growth models do suggest that number, it happens to coincide with rough estimates for sustainable peak population. An argument can be made that we became overpopulated the moment our waste products began to overwhelm the environment, but it's not a popular view. It's more common to define overpopulation as the point where we begin to struggle to feed ourselves... and I think that starts to become a serious concern around 12 billion.

Musk's concerns are the wrong way around, imo. While the earth is our only home, we should celebrate any slowdown in growth.