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.
Earning merits do not matter the number of post you make a day, but it matters much on the quality and usefulness of your post that even a single post can earn multiple merits.