[AChimp]

Learn a bit about distributed systems. Guild Wars is P2P. How else do you play with other people in your instance? When you visit a town or a place with a lot of players, you are then playing on one of the central servers.

This way the company only has to maintain a small number of machines since only a small portion of the players will be in the towns at any given time. Everyone else is communicating between themselves, and only with the people that they need to.