Activity is calculated according to this rule and is updated every hour.
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)
Once you reach 14 posts and post more, it will be updated once on the Wednesday after you make your first post in the next week.
In other words, wait until next Wednesday and you will see that your activity has been updated.