The minimum post to earn merit is one post. You can make one post and earn several merits if the post is very useful and helpful. You can make hundreds of posts and no one will send merit to the posts because they are useless or of low quality.
Read what good posters are posting and start being like them. Post something useful and help.
You're absolutely right only one post can make one to earn merit depending on how quality and meaningful your post is, there are some people who made over One thousand post but no single merit that was given to them. reason is because there post lack so many qualities this is why most people find it very difficult to earn merit from their post, something they keep acquising those high ranked members for being greedy, without knowing that there post lack so many informations.