Depends how many players / city.
Small town: 3 players at one time
Town: 4 players
City: 5 players
Metropolis: 6 players
So, 5 cities of a size:
5 small towns: 15 players
5 towns: 20 players
5 cities: 25 players
5 metropolis: 30 players
We can then support 5 metropolii on a 32 player server and have room for 2 unaligned players, or 5 cities and 7 unaligned players.
We could have cities belong to a larger nation as a whole and separate further:
Metropolis + 2 small town = 12 players / faction
2 factions = 24 players, 8 unaligned
Try this then:
The Big Three:
The Underground City
Metropolis / 6 players
The Floating City
Metropolis / 6 players
Bushimenentown
Metropolis / 6 players
The Towns
Third Party 1
City / 5 players
Third Party 2
City / 5 players
(Alternatively)
Third Party 1
Small Town / 3 players
Third Party 2
Small Town / 3 players
Third Party 3
Town / 4 players
Unaligned
(these are examples; could be anything)
The Traders
Travelers / 2 players
The Mercenaries
Travelers / 2 players