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’ve read Harry Potter countless times, I used to read the whole series every summer break, it was just so comforting.
Apart from that I’ve also read “To kill a mockingbird” a good few times, I love the story, the narrative and the humour !