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Most probably on 1036.

There are plenty old accounts that have "banked" activity. I would not be surprised if some of them could reach legendary just by posting.
What do you mean by this?
Old accounts which is unused may have posts greater than activity. Activity of these accounts may increase by a lot by posting. CMIIW.
Not really correct, if an account got the post count higher than the activity, the account will automatically get 14 activity despite that he only posted once in 2 weeks period
What it meant by banking activity is that posting once or a few post which is less than 14 in a 2 weeks periods, since the post count is low, the activity count cant be higher than the post count
The activity count will then be "banked" so that once the account posted some quite some posts, the activity could get higher as well, you can actually see this with an account that got the same activity and same post count
EDIT :
I am member here since the end of 2013. It seems I have more posts than activity, the difference is 139. Does that mean my activity is not limited and I can increase it by 139 in a few weeks or months?
You can only get 14 activity in 2 weeks period only if you posted at least once it the 2 weeks periods, so you will increase your activity by 14 in the next tuesday