Not sure someone with knowledge will give an exact answer, but if you look at the board you can get an idea. There is someone on there with only 20 reviews, so that's a good starting point. I checked a few city boards, and no one under 20 was listed.

 
Also, there is a second criteria. There needs to be recent reviews. Not sure how recent and if there is a quantity involved in this equation. But I have seen escorts drop off the list who still rated high but hadn't had recent reviews.