Don’t take my word for it, the latest research confirms that JWs generally underestimate membership, Mormons grossly overestimate membership and Adventists are pretty accurate.
Worldwide Adventists are the most numerous, then JWs, and Mormons trail a distant third.
Lawson, R., & Xydias, G. K. (2020). Reassessing the Size of Mormons, Adventists and Witnesses: Using Census Data to Test the Reliability of Membership Data and Accounting for the Disparate Patterns Found. Review of Religious Research, 62(3), 413-437.
https://link.springer.com/article/10.1007/s13644-020-00408-z
“This study has compared official memberships of Mormons, Adventists, and Witnesses with estimates based on self-identification in census data from 54 different countries and a census proxy estimate for the United States. Official data show that the Adventist membership is the largest of the three groups, followed by Mormons, and then Witnesses. However, while the census data also show Adventists as the largest, Witnesses are second and Mormons trail as a distant third. The US census proxy for Mormons is the only category where the official data and the census proxy show reasonable agreement. For other countries, Mormon self-identification is less than a third of official statistics. Self-identification for Adventists exceeds their official data by almost a third, while those identifying as Witnesses are more than twice the official numbers. These data demonstrate that official membership statistics do not necessarily provide a reliable way of quantifying size: close attention must be paid to what categories are counted as members, and how accurately membership turnover is acknowledged.”