The number of posts doesn't count. Patchou chooses his beta pesters according to their knowledge about Plus, and they behaviour on the forums. If you help a lot in the forums, and understand all or nearly all of the features of plus, in the future you can become an official tester. However that doesn't happen always, due to several reasons: Patchou doesn't need more testers, he doesn't trust you, etc...
Anyway, don't try to become a beta tester. By asking you won't become one, sorry
.
/ 
