A very rough estimate of 20,000 copies sold is reasonable. It might be more; I’d be surprised if it was much fewer.
I have to disagree for a few reasons; while it's interesting that he's gotten 600 reviews, I think more readers of his book would be impelled to review it because they were more excited to see a book against Watchtower than, say, someone reading a random mystery novel. His followers are also more cult-like than you find with anyone else so they'll leave a good review if their lard and master tells them to.
Bookstores don't sell self-published books as they don't work with individual authors but through a distributor. Distributors don't work with self-published authors as it's a nightmare trying to establish revenue sharing, returns, etc. Remember that dumb little book signing he did in London or wherever, no one showed up and he left a few with the store owner?
So we can rule out a lot of members of the general public buying it. That leaves exJWs on social media, Hemant Mehta's audience (he wrote the foreword), random YouTube viewers, and anyone clicking on his website.
Also, the size and price is prohibitive for a lot of people and there was a free PDF floating around for a time.
So I'd say several thousand is a good estimate but highly doubt it even got to 10,000. Just my two cents of course.