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=237597.0

The 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.