Yes, I ask often to me this question! I'm here since 2 years and I've some sales, not enough for me, but it's never enough, no?! ;-)
And I'm also in more than 10 other microstocks websites with between 340 to 550 pictures, it's depend of the site.
With the totality, I've some sales each month, but how many pictures does it hopes to do his full time job?
1000, 2500, 5000, more than 10000???
What's your experience and/or your opinion on that?