Of course it supports so many players but you will need ANet Professional for this (I'm currently working on it). But it's not ANet, that will limit you to less players, but you won't have that good servers and internet access.
Just imagine what traffic 250 players would produce

Normally you should use zones for that much players (like in common MMORPGs). You have servers for each zone and switch between them. This is supported!

You can try out the free demo. The link is in my sigantur.