Well, being a newbie and trying out the search function I've found the answer to my own question in this link:
https://bitcointalk.org/index.php?topic=237597.0The activity number is determined in this way:
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
Activity is updated every 30 minutes.So basically the more you post and the longer you are here, the higher your activity number. In the first two weeks it cannot go above 14.
You got it! There are different member status that come along. Some have features such waiting less time between post's. But just have fun on the forums, and it will go up slowly.