If AI works well, we'll reach the singularity and nobody in this world knows what the hell will happen.
We might merge consciousness and live in an ethereal medium, we might insert ourselves in a simulation and keep our physical bodies neatly healthy organized and packed where 20 Bln is not a problem at all, we might collectively (or AI do it for us) decide to reduce human pop to 1 Bln or less on Earth and terraform other planets.
Call me crazy, but if you described our world to someone some hundreds years ago, theyd think the same. Crazyness is a requirement for a correct prediction of the future. There are ALWAYS status quo changes based on revolutionary magic-like technologies that results in significant changes.
If you want to have at least a mild change of doing some sort of future prediction you can check predictions about our current time from people in the past and you'll see the mistakes they make, most people nowadays keep making. Using current technology and ways of thinking, as if they would stay the same just used in different ways, not thinking enough out of the box and allowing themselves to be more imaginative. Fix that, and you'll make wild predictions but also some of them which will be much closer to the truth than any status quo linear evolving incremental improving technology based prediction.
In reality by 2100 the world's population will be around 10.2bln and declining from 10.3bln in 2084 or so.
And that also assumes the UN won't revise its projections once again, as every time they did, they had to lower their estimates.