Actually given a fixed structure depending on how they limit player by server will drastically change performance:
Let's say you use all your structure to create a single server, everyone see each other, the network load is huge because the server has to transmit every player position to every other player. It won't work.
Now let's look at the opposite case, you create a single server for each player, the server also has to manage NPC, and their behavior, pathfinding reaction ect. The server is not able to able to handle THAT many world at once either because of the insane number of NPCs and world related data.
Finally between the two there is a middle ground when handling world data (fixed amount), and the player data (which scale exponentially with the number of player) which yield the best result. That equation is obviously even more complex given player density, max player view distance ect.
Given the overflow system, that threshold, between number of overflow servers, and number of player on overflow can be adjusted on the fly. (Which has never been the case with previous MMO).
Finding that threshold, can be actually very difficult and even with numerous stress test, could only be estimated before release.