Assuming you do know we are currently only using 3% of the Earths land surface, what reasons are there that humans will run out of space?
Could live vertically, even above ocean if tech allows it (with instant transortation maybe.). Theoretically we have some good amount of room left.
At even midsize population density, I am pretty sure we have space for WAY over 100 billion people on this planet. Food production needs some more work, perhaps a bit of us eating lower on the food chain..