Obviously Mio have more than 2000 fan worldwide, if not much more.
Don't have solid proof, but take those 100 fanfics
Ota-kun just post (can you please delete that Mugi picture pls, you knows which one...) there are only 37 of them don't have Mio in it, and only around 10 more are group pictures. The total of fan-art for the past few days have passed several thousands = few thousands fanart of Mio was produced.
Then if we consider every 10 fans , one of them will draw something and put on the internet, then the number would easily be ~ tens thousands if not more..
Actually nvm about whole that assumption..... i just say it because every game+ anime+ manga forum/site/blog i have passed by have K-on related articles/posts/thread in it, with Mio signature and avatar fly around.....
Amazons sold ~3000 of ED single as well
...and we didn't take Japan into account yet....