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 have to say terribly Twilight 13 times, i reread it every time there was a new book coming out and i was kind of obsessed with the book as a preteen. My paperback copy literally fell apart one day.
For adult me Outlander i have at least 6 reads of since my first read at like 16. I only got through book 5 and every time i want to read more of the series i start totally over and never end up getting past 5.