I have a job to put a figure on this, as some have said, exactly what we all feel is a " true believer" is also problematical, it is so difficult to determine what "doubts" an individual has, and how deep they go.
In my case I was 2nd generation, baptised at 12, served as a regular pioneer for a while, a MS etc in my nearly 60 years in. And yet I never believed ALL of it, I particularly never bought in to the Armageddon ideas they had, or in to, more importantly, the Cult of the Governing Body.
From the outside you would say I was a " true believer", but I was not, not in the way the Governing body would want me to be. I guess what we are trying to measure is " Depth of Delusion".
Those that are sufficiently deluded will follow straight away without question whatever the G.B says and whatever direction they go in. I would take a stab and say that is in fact only about 2 million of them or even fewer than that. The next level kind of tries to believe it all, but doesn't fully, then you have the half awake, then PIMO's, but stuck in , and then those that are really fully awake too, but are not stuck in, just in by choice.
Before long the lower 6 million will start to drift away. 2 million will remain, in a mainly on-line religion. My estimate.