Todays 'mmo' games are the ones that are made mainly for multiplayer without single-player content and can support big number of people by either
1) having a official server (or servers) that hold to hundreds or thousand players at once in the same 'world'
2) have multiple user created gaming rooms that hold to 20-80 people at once (most korean mmos)
From the information presented about World of Planes so far i deduct that WoP will have second option but there is also info about an 'online war that can last a few months' which can tell us that there will be also 1st option
or i can be totaly wrong and we will see something different