"The servers" is the vague part of your question - depending on how you set up the world, you can expand quite a bit. EVE Online has a single-shard setup that uses a massive server farm to run, Guild Wars has entirely small (population-wise) instances, WoW is mostly non-instanced common areas with some instances, Age of Conan and City of Heroes have loading-screen transitions between areas - there are nearly as many ways to build a large world as there are worlds.