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.
Shadow of the Wind by Carlos Ruiz Zafon. Once when I was a teen and another when I was a young girl adult (still am) and just in those few years you understand things and characters so much differently. Great read highly recommend.