There were a lot of bison all over the park. We read that in the early 20th Century, the herd in Yellowstone was down to about 20. Some breeding stock was lent to the park at the nadir of the herd, and it there are now two or three thousand bison there.
When we saw them, these were slow moving creatures. In fact, we often mistook glacial erratics (boulders deposited by the 4000 foot thick glacier that once covered the much of North America) for bison.