How reliable are JW figures? Ronald Lawson and Kenneth Xydias did an impressive amount of research on this topic and published their findings recently. They examined census results in 54 countries, as well as five surveys in the United States, and compared these with official numbers from the Mormon church, Jehovah’s Witnesses, and Seventh-day Adventists. What did they find? They discovered that Mormons grossly overstate their membership and Jehovah’s Witnesses vastly underestimate theirs.
They conclude that while the Mormon church claims 16.1 million members, census and survey data indicate that only 8.6 million identify themselves as Mormons.
Jehovah’s Witnesses claim 8.2 million publishers, but census and survey data indicate that 17.5 million self identity as Jehovah’s Witnesses.
Seventh-day Adventists claim 20.7 million members, which was also an underestimate compared with census and survey data that showed 26.8 million.
https://link.springer.com/article/10.1007/s13644-020-00408-z
So on the basis of that research, Jehovah’s Witness official numbers give a modest representation of their numerical strength.