Today's video from the Isaac Arthur YouTube channel examines whether a trillion people can live on Earth. That's more than 100 times the current world population. If the population grows at its current rate and doubles every 50 years the population would reach one trillion within 350 years.
With today's technologies the video explains how we can accommodate a trillion people on Earth but the standard of living might become lower. With advances in energy generation/storage, automation and biological modification we would be able to sustain a trillion people with living standards better than modern middle-class society. Virtually a post-scarcity society.
Besides fusion one of the energy sources that could help power a huge population like that would be satellites that convert sunlight into microwave radiation and beaming it to antennas on the Earth's surface. Besides possibly having more satellites some of the population may choose to live in orbit or another planet before Earth's population reaches one trillion.