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.
Harry Potter and the Sorcerer’s Stone.
And it will probably stay that way forever. I’ve read the series a couple of times, when I was younger and the first one used to be my comfort book for years. If I’m including listening to the audiobook, then I’ve read that one at the very least 10 times, probably more like 20 times.