IMO, the number of "True Believers" is academic, since - as has been demonstrated here - its definition is so fluid.
"Loyalists" might be a better focus.
And this should be a little easier, simply because all you have to do is make a guess based on your own congregation (assuming you're still in one).
If you know approximately how many people are in your Hall, and you have a pretty good idea how many full-on loyalists are in your Hall, it's pretty basic math.
Then, simply view a JW congregation as the Org in microcosm.