You don't need a certain amount of posts at all to use the Private Messaging system (as far as i know). It's that way on most websites. But you do need to have 10 posts at least before you can use an Avatar from this website that goes under your screen name!
But now that you have 11 or more posts your private messaging system should work ok for you. Just click on a person's profile and click on Send a Private Message (to that user's screen name) to type what ever you out and click on send private message. And then once that person replys back to your message click on my inbox to see your private messages that you have sent to other people and private messages that other people have sent to you.
But for the ***Record again*** you can use the private message system weather you have 1 post or 500 posts