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.
Intensity by Dean Koontz, when I was a kid, the outsiders by SE Hinton, and I’ve reread the book Son of a Grifter by Kent Walker quite a few times.