If you make 14 posts in each 2 week interval then you will gain 14 activity. You can't gain any more than this.
If you make less than 14 posts and your post count and activity are the same, then your activity will increase by the same amount you posted by. If your post count is greater than your activity, your activity will increase by 14 each interval so long as you make a post.
You can see the date each activity period starts
here. You can see how much activity you need for each rank on MZ's link above.