Take a look at the pic below. Anyone who played Test Drive Unlimited 2 will know what i'm talking about. Even though each server will limited to 16 players, users would regulary switch servers depending on where abouts they are on the map. The transition will be smooth and the user wouldn't notice the server change, much like in TDU2.
A system like this gives the user the feeling that they're in a world with hundreds of other players, yet they're actually only with 16 players but just switching to a different area on the map. This would fill the map and utilize it as many people would like.