In short, as many as you can get. There’s no such thing as too many reviews. More reviews link to more data on books read and sold. The law of large numbers is at play.
Lamentably, though, many self-published books don’t get any reviews–or at least legitimate ones. Too many authors focus almost exclusively on the writing and not enough on the marketing of the book. Of course, you can always pay to play.
Publishers Weekly offers something called PW Select. You pay $149 for the chance to have your book reviewed on the PW site. Note, though, that there’s no guarantee that it will be reviewed or that said review will be positive. Kirkus Indie charges authors a fee of roughly $400-$550, depending on the speed desired for the review.
All else equal, more reviews mean more sales. Paradoxically, even bad reviews can help sell books.
The Age of the Platform has garnered nearly 100 Amazon reviews and I want more. At my speaking gigs, I’ll sign books and politely ask people to put honest reviews on Amazon.
Reviews help sell books. Period.