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 don’t really reread books now, but as a child, I reread a lot of my favorite books - The Hobbit, the Mummin series, Winnie the Pooh in Russian (more of a retelling than a literal translation), the Captain Blood books, Speaker for the Dead by Orson Scott Card, Jack London’s many short stories, Agatha Christie, Simenon, and Conan Doyle.