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)

For example, if in a 14 day period you post 14 posts, you gain +14 activity.
If you post 2 posts in a 14 day period, you gain +2 activity.
If you don't post at all in a 14 day period, you don't gain activity.
However, if you post 15 or more posts in a 14 day period, you still gain only +14 activity.