All the rules are written in the first post of this topic.
Well, I've made multiple threads which have hundreds of replies about gambling, economics, sports etc yet only have 5 merits.
I only registered my account three weeks ago so maybe I'm being too hasty.
It basically doesn't depend on the amount of replies you get on the threads you've started, but it's more about the quality served from the threads you've started. Also, people with different tastes and choices give away Merits based on their personal views and how constructive they find your post to be based on their own scale of merit-worthiness.