question : can you subscribe multiple times on one disc?
or how does it work, i mean the game isn't sold 10 million times right? right?

 Nope, you can't. One account per disc.

Also, this total doesn't include inactive subscribers -- people who used to play but no longer do -- of whom we can assume there are millions.

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)

 basically 10 million people have currently paid some sort of fee for the game, so you pay as well say 10 million people currently own & play wow....

However, if you include people who no longer play, who knows how many people have paid ! 

Nintendo and Blizzard should team up since they are both very good at making money.