I don't know any hard stats, and I can't imagine there being any good ones because it's highly stigmatized. Check out reddit.com/r/detrans and detransitioning in general. Triggernometry has a couple interviews with detransitioners too.
Discussion
I've also found several books on the subject that I plan on reading, but I haven't yet:
Lost in TransNation by Miriam Grossman, M.D.
Irreversible Damage by Abigail Shrier
Trans by Helen Joyce
These should have citations that point you to what you're looking for.