Just a little correction, you will only get 14 activity in "2 weeks" no matter how many extra posts you make. Just make sure you have at least 1 post per 14 day period so that your 14 activity points get accumulated with your account and they will increase as you post further(irrespective of the time you take).
Sorry, thanks for correction
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)
~iki