Why don’t you believe there are 8,000,000 JWs?
JWs are leaving in my experience, but at a far slower rate than other groups.
For example there were around 500,000 members of the Church of Scotland 20 years ago. Now that figure is down to 325,000. That’s a pretty steep decline.
For JWs, they used to get around 9000 people at their district convention in Scotland 20 years ago. At the last convention in 2019 the number was pretty similar, possibly down slightly, but still over 8000. Plus JWs have added a few Polish congregations and other small groups in Scotland in the meantime. (They presumably attend a different convention, but still count as JWs in Scotland) JWs are pretty much treading water in Scotland, as elsewhere.
Every independent census I have seen shows that JWs underestimate their membership compared with official counts, sometimes by a significant margin. For example, official census figures show over a million JWs in Mexico and Brazil whereas the Watchtower publisher numbers are below a million. So there are not 8,000,000 JWs because by measures comparable with other groups there are far more than 8,000,000.
When evaluating JW growth rates there are two ways of looking at it. On the one hand their growth rate is very poor compared to their own growth rate in the past (5 or 6% 30 years ago down to between 1 and 2% in recent years). On the other hand, most other churches are declining rapidly (many churches declining by 1% or more per year).