Has it anything to do with the number of posts
Yes.
or is it all about how long you have been a member?
Yes.
I have a question which I cant seem to find the answer to.
Newbies - Read before posting How is forum activity calculated?
Activity is calculated with the following formula:
activity = min(time * 14, posts)
Activity is increased by staying logged in or by posting and is updated every 30 minutes. There is a maximum of 14 activity points every 2 weeks.