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.
Demian by Herman Hesse. Although it’s not as impactful to me as the first time I read it, I enjoy the new interpretations I have with each read. I would find phrases that never struck me before but do when I read them. I pick it up almost yearly, and the annotations I left are sort of a great marker of where my head was at and see how my perspective changed.