MMO Map number and size

ok so im planning to make an zombie MMO with some friends where players can choose to play as either a zombie or human. My question is should i have multiple scenes, each representing a different city/district/whatever and let players choose which to play at or should i just make one and not worry about interiors and whatnot and how big i should make the maps. I of course want the game to be fairly large but i want to pick the way unity could most effectively load/run. also as a side question will the photon bootcamp demo modification on the asset store be able to handle this or will i need a different server? thanks in advance