The source is a training book on how to sue psychiatrists, I can't find the study anywhere. Even if it does exist, it's saying that a lot of therapists have seen a client who had sex with a therapist. If someone wanted to have sex with a therapist, they would visit a lot of therapists.

This explores the topic in a more meaningful way, and links sources. https://kspope.com/sexiss/research5.php#a8studies

I assumed the study is referenced in the book (and that the quote is from the book) -- but honestly I have no idea