If you want to be pernickety then they are no where near the number one spot. There are a few Asian mmos that sit at the 20 million mark. If you use Blizzards own definition of sunscriber....
Subscriber Definition: World of Warcraft subscribers include individuals who have paid a subscription fee or have an active prepaid card to play World of Warcraft, as well as those who have purchased the game and are within their free month of access. Internet Game Room players who have accessed the game over the last thirty days are also counted as subscribers. The above definition excludes all players under free promotional subscriptions, expired or cancelled subscriptions, and expired prepaid cards. Subscribers in licensees' territories are defined along the same rules.
While it may be absurd to call them subscribers for playing a free game the most prevalent way of playing mmos in Asia is in game rooms. By Blizzards own definitions and just to wind up BenBros they can be called Subscribers. Thus to name Age of Wushu that has 20 million players in China alone it is the King and has been for several years. If Blizzard counts them as sub i guess any game can. Amiright?