activity is based from the ranking of your position in two weeks the activity will increase in 14 or 12 to but the posts are just how many did you post in your account their are not limits from it unlike the activity and also they are the same because its a delay or a bug from the website or in other reason they just hide it well 
Sorry, this incorrect. And the first member that replied to you offered you the correct source.
But to Canah here, you earn 1 activity point for each day you're active on the forum. "Active" in the forum requires you make one post every two weeks (e.g., 1 post in a two week period earns you 14 activity points).
And every two weeks your activity will increase accordingly to the lesser of the number of days you've been active or the number of posts you have.
Welcome to the forum!