Near the beginning of Star Trek Beyond, the Enterprise docks inside the new Yorktown base. As it's shown flying inside we see numerous support structures, each of which seems to hold a large number of what I can only call "skyscrapers", though given the setting that term seems to not quite fit. As we only see a small part of the base in this way, the full base could potentially hold enough space to dwarf any major city now on Earth. But is there any indication for what the population actually might be?
"Millions", according to Krall.
Sulu: You're going to attack Yorktown.
Krall: Millions of souls from every Federation world holding hands. It's a perfect target.
Star Trek Beyond
Unfortunately, he doesn't specify how many millions.
If you look at a scene where Kirk and Krall fight, you can count 100 buildings per column. Multiplying by 6 columns and 1 million per building, you get nearly 600 million souls.