I like there to be a lot of people, but not ridiculous amounts. Preferably a few thousand at the most.

Although some games i make exceptions, such as NationStates. There are 1.6 million people playing, but once you join a region, you'll end up being nucleated into that region and it's alliances. I'm a resident of India, and currently we're allies with about 6 other regions, of which we're the biggest, with 70 people. We're fighting against raider organisations who attack small regions. All in all there's probably about 1000 people involved, and it seems like epic power struggles, but when you look at the rest of the game, it's pretty small on the big scale of things.