That's true but pretty much impossible to "guess" the number, level (do you count everyone who ever touch the TT bat? Or just these who do it at least once a year? Or only those who have decent level? And what is that level?)
So the only thing you can somehow "count" with certain precision will be registered players under national associations. That is pretty easy for developed countries like those in EU, US, CAN... (at least those in Central Europe have pretty opened registers so you can find the information, although sometime only in native language). For countries like China, Korea and Japan I have no idea. E.g. are these metropolitan "leagues" and "tournaments" (like Beijing and Shanghai) somehow linked with CTTA? Or are these some separate registers? I would say these big associations will keep track on those numbers anyway so you should be able to get it but again probably just in native speaking sources. Good luck! (try to contact some active members here from HongKong, Japan, Korea, they might be helpful)