I guess it depends on which style of mmo they go for. Not entirely sure how popular western mmo's are in the east. I know wow has a foothold in china/taiwan.
They have almost 200 million mothly active users and usually have a daily peak player in tens of millions. So, it's safe to say if they can get their LoL players to switch over they could have peak WoW numbers, easily. Whether or not they can sustain that is tbd.