The are millions of Mormons, hundreds of thousands of JWs in the US and no reliable numbers on Scientologists have ever been objectively verified.
Articles examine this under group subsections.
Many groups do seem to inflate their numbers.
Scientologists say they have millions of members, but critics say they have around 100,000 to 200,000.
Another example would be the Kabbalah Center, which claims to have a huge global following. However, at main worldwide events they actually draw only about 3,000 to 5,000.
There are millions of cult members, but no exact count has been taken, checked and verified for certain.