Does this mean that the activity depends of the number of posts?
No, the activity increases with the number of posts but it is capped to 14 activities every 14 days.
So even if you make more than 14 posts per period your activity will not increases more than 14 per 14 days.
I, as a newbie, currently have 6 posts, but only 2 activities.
Some explanation.
You need to wait a little bit to see the activity changing.