Warhammer: 1.3 million subscribers

Serial Ganker has been doing the math, and estimates that Warhammer Online (or any other game entering the market) can attain about 1.3 million subscribers. Western subscribers, that is. The majority of WoW's 10 million players resides in the Far East, where Warhammer won't be released (at least for now).

However.. there's one disrepancy. As Green Armadillo pointed out, WoW has 2 million players in Europe and 2.5 million players in North America/Oceania.