The Mormons notoriously inflate their numbers. Scholars assessing dozens of census results concluded there are 8.6 million who self identify as Mormons compared with 17.5 million who self identify as Jehovah’s Witnesses, making Jehovah’s Witnesses more than twice as numerous as Mormons. In addition Mormons are far more geographically concentrated than JWs in a few regions such as Utah, South America, and certain pacific islands. (The same study found that there are 26.8 million who self identify as Adventists worldwide.)
https://link.springer.com/article/10.1007/s13644-020-00408-z