Unless all one does is read fiction, I don’t think hitting seriously diminishing returns or running out of top-tier material are likely problems. Mere mortals with other interests and hobbies who achieve low-tens of books a year, and not all in that meaty very-high-quality bunch, need not worry.
Why not read into a nonfiction topic just for interest's sake, like a layman's accessible book on quantum physics, or the battles of WWII, or the development of Western music, or whatever topic makes one just go "Hmm, that's cool, I want to find out more about that."
So yes it's obviously not infinite, but I imagine it's at least 10,000 and that's enough for anyone.
As long as you keep changing, the quantity of books to read is infinite. I like the analogy of the article that books are like a stream and just dip in and pick the one of the moment and let the rest float by. It’s only people who don’t change enough who consume too much from the same point in the stream and feel the effects of water sickness.
https://www.amazon.com/Million-Random-Digits-Normal-Deviates...
Though actually. Stopping to think about it now for the first time I think I'm about there at this point. It has definitely changed what sorts of books I seek out and what I expect to get from them. But I still find reading fiction rewarding.
I worked in a video store for years and watched many thousands of movies as well, and while I am almost completely burnt out of watching movies because there's so little innovation going on I could not imagine that being applied to books, the direct interaction with someone else's mind is too interesting.
The only problem with modern books is there's so much trash along with the treasure, there's never been a lower bar for self publishing.