Even though you will do dozens of posts, your activity will increase by maximum 14 every 2 weeks. So don't try to spam and increase the activity, it won't work that way. Rather do some useful and thought provoking posts so that your activity will increase and you might get some merit too (merit will be required to update your rank after Jr. Member).

Regarding limitations of posts, it varies as per your current rank. Newbies can post a reply after minimum 360 seconds from the last post.