that's the number of posts you make on the forum. the more you make a post then the number of your activities also increases
This is not the number of posts. Posts are required to increase the number of activity points, but you also need time to do so. During the two-week period (because activity points are updated every two weeks) you can get a maximum of 14 activity points.
In other words, if you write 40 posts within 2 weeks, you will receive a maximum of 14 activity points. If you write less than 14 posts in two weeks, you will get as many activity points as many posts you write.
Activity points are required to be promoted to a higher rank on the forum.