When should a mobile app be launched?
Well mobile app is already kinda launched but its still in closed beta or something.
Do we know how many people got beta invitation?
I don't think the team posted this kind of information - it would be great if they could share it though...
Given the regional restriction I don't believe that a lot of people were involved in testing.
Which is the optimal number to do such Beta testing?
Anything from 1 to whatever is acceptable

because it's free so any help is valuable.