It didn't work with 600.. certainly won't work with more. So frustrating.
There are various limits that the game world hits at various times:
1. Limits of aircraft on the UM
2. Limits of what production lines can produce
3. Slot limits at slot constrained airports
4. Overall passenger demand
There is not a huge difference between 600 and 700 players. Eventually, the gameworld loses players as some players take up more of the limited resources above.
In theory, all these resources could be indexed to number of players in the game world...