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.
Catcher in the Rye. 4 times, 4 different books, 4 different decades. Have yet to read it , now 53, in this decade of life. First time at 16ish, again in my 20s, 30s & 40s.