There is no "post requirement" to be a Mod. I've seen Mods with <1000 posts. But, generally, the vast majority of the Mods will have 1000+ posts, because the staff has to know you and your style of posting, etc. to consider you.
The theory that if you have a certain amount of posts you're likely to get Mod is not true. Look at the people who have 6000+ posts and still not a Mod.
Jed made a good explanation of the process of becoming a Mod:
Originally posted by Jed
Usually, you're a prominent community member with a history of responsible posting. If a spot opens up or there's a time where the staff feels like there is a need for more moderators (influx of new users, time around a new game being released, etc), you're contacted by a member of the staff and asked if you'd like to join the team to help out.