As far as I know the system is still a cluster system at its core (but an, essentially, unlimited one due to the cloud aspect of it). They do not do a good enough job at describing its limits on their website I think (no surprise there, they are about selling their idea after all) but I'm fairly certain each server in the cluster handles a specific area of the gameworld and you then have special edge case server to server information set up when players are changing area (and thus server).
The above works fine for most multiplayer games where the population of players will be spread out across the entire game world. If you run into issues with a server/game area being full you can then assign another server to the area and create an instance of it, etc. etc.
In a game such as WoR that will not work as our game is centered on large populations of players in tight groups/formations - I'd love to be proven wrong though.
One thing is the server side performance - we could probably bump our servers up a bit in cap - another is the client side performance however (this is our bottleneck currently): the more players the more playerlogic code your client has to run the bigger a CPU hit you will take.
EDIT: I just noticed this is posted in the regiment suggestions - hardly the right place.
- Trusty