Not including short story collections, poetry collections, anthologies, etc.
For me, I believe it’s Oroonoko by Aphra Behn because it was on the syllabus for three different classes that I took in college. Other than that, the most I’ve read a single book is twice, and that was Telegraph Avenue by Michael Chabon. I’ve read Dracula 1.5 times. Didn’t finish it the second time around.
I read Harry Potters many times as a kid, I can’t remember how many times. Enough to memorise entire paragraphs. Since I was a kid while it was being written and released, and even after the release we needed to wait a little longer for the translation, it made sense to reread the previous books.
Other than that I think my most reread book is Pride and Prejudice. There’s something so comforting about this book that when I’m overwhelmed with life, I find myself having gone back to it. I found a similar comfort in Tenant of Wildfell Hall, so I think that book will also be reread many times in the future.