I think 20 is the correct answer because I have seen 20 posts in EMP rules area.
Where have you seen 40?
By the way, Just yesterday I received my first payment from EMP 5.24 upon 2 hours of request. That's really fast.
I found the 40 daily limit in -> forum rules , a thread start by moderator stefgrid, call advance posting rules just recently posted on 7 Dis. Some details as below :

5. PAID POSTING RULE

* Our forum is a paid-to-post forum. So do not try to increase your posts by just posting a couple of smileys or short messages .Strict actions will be taken if a member is found doing this.
* You are allowed to make only 20 posts/threads per day.
* To check your daily average posts go to Profile-> Statistic. They must not exceed 40 posts per day.
* Posts made in the area "Exchange Money Place Community" are not counted.
* All posts are checked for their quality, before payment.

Congratulations on your payment received again!