A Mormon congregation in Los Angeles, California. Photo by Dale Wight.
Different churches have different ways of measuring their membership, so it’s not always productive to compare religions this way. Also, some religions don’t have a central governing body, so any small congregation can claim membership in the larger group. For example, many Baptists would not consider the Westboro Baptist Church to be Baptist given its offensive beliefs. But that church does have the word “Baptist” in its name, and they do consider themselves to be Baptist. Do they count or not?
Given this confusion, I’m not quite sure how to answer your question. I can say that LDS Church records indicate that there are currently 15.6 million people who are registered as members of the church. I hope this answer helps.