Someone please correct me if I'm wrong but I think it carries over to the next month. You should be alright as long as your name is on the currently enrolled users list. So if you haven't reached the 50 post limit on the 17th of a particular month, then you can claim it on the 17th of the following month. If by then, you still haven't reached the limit, then you can wait however long it takes until you reach it and then claim it on the 17th of that month.
Thanks for the clarification !! do i need to process payment manually ?? cause from past 2 months i didn't processed it once .. So my post counts are still acceptable for payment ??