actually the majority of people in china DO NOT own a disc and infact play at a local internet cafe. The just pay a subscription fee (vie top up cards).
However i bet atleast 12-13 million have at one point played the game, and its easily one of the most purchased of all time.
And even in the US/EU you do not have to own a physical copy of the game (you can pay online for a code).
Its hard to keep track of games that have high levels of digital sales such as WoW. however even without digital sales, its probably sold 5 million box units (minimum)