Yellowstone park has Bison and not Buffalo as far as I can remember.
It's been a few years since I've been to Yellowstone national park and the last time I was there I could only remember seeing Bison and no Buffalo.
Although sometimes Buffalo and Bison can be confused so some people may see them and think they are Buffalo when actually they are Bison.
It's estimated that Yellowstone has a population of at least 5,000 Bison in the Park and surrounding areas but the Population may be more or less because there's no way to know for sure 100 percent.
If you've never been to Yellowstone I highly recommend you go to Yellowstone as it's a great place and great experience.