Over 100 million
Callisto is the fourth Galilean satellite outward from
Jupiter and is home to a large population of citizens of The Rim.
Callisto is the second largest of the Galilean moons orbiting
Jupiter and the furthest of the four. Its radius is approximately the size of Mercury.
Like most of
Jupiter’s moons, Callisto is highly militarized by The Rim. It is likely the  most militaristic, since the Society docks a major garrison fleet there and it is one of the breeding grounds for  Grays and Obsidians. 
The Second Moon Lord Rebellion Edit
At the beginning of the Second Moon Lord Rebellion, 11 days after the
Triumph of Darrow, members of House Raa stole or destroyed the entire societal garrison fleet at its dock in Callisto. This event spearheaded the Moon Lords into declaring independence and electing Romulus au Raa Sovereign of The Rim. 
Towards the end of the rebellion, Callisto is one of the last moons to be taken by
Roque au Fabii and his Sword Armada. Ganymede and Io were the last two moons unconquered.
Various buildings Edit
There are 8,000
Peerless Scarred between the four Galilean moons.
2.0 2.1 Morning Star - chapter 10
3.0 3.1 3.2 3.3 Morning Star - chapter 40
↑ 4.0 4.1 Golden Son - chapter 25