As many as you think positively contribute to the forum - there's no fixed limits. I wouldn't concern yourself too much with it though because you're already banned. By making a duplicate account you've just lengthened your punishment. Unlucky!
You don't get banned for not posting. I suppose any forum from time to time might get rid of users that haven't been active for x amount of months.
Actually, I think users who have been here since 2004 and inactive still have their accounts.. they're not deleted.
I think in your case the more you post, the more likely you'll be banned. J/K. Kind of. No really, I am. Maybe.
I don't believe DP "Pruned" any thread And post as much as you like.. just make sure they aren't spam and are actually meaningful.
Shhhh - I told you not to mention that Joe! But seriously - I just have a very good memory. His namesake got banned for running his post count with nonsense - whad'ya know the coolsalman_12 version comes along asking about how to avoid being banned for the same thing again in future. Smells of fish methinks!
I guess you didn't produce enough as you have been banned. It is not about posting, it is about contributing good posts and not breaking the forums TOS.