There are 7 campgrounds in Yellowstone that don't even accept reservations. Therefore, many people go there without reservations.
There are 5 campgrounds in Yellowstone that accept reservations.
I'm not saying you are guaranteed a site but this is just our experience. We've stayed in Yellowstone with our 40' motorhome about a dozen times and never had reservations. We've also stayed at Madison and Bridge Bay (reservable campgrounds) and didn't have reservations. We got sites because of cancellations.
What you need to do to get into a non-reservable campground is to stay outside the park nearby the previous night and move into Yellowstone early morning when folks are leaving.
We've also gotten sites in Baker's Hole national forest campground (not reservable) just outside of W. Yellowstone. It's a beauty right on the Madison River.
We rarely traveled with reservations in our 16 years of full-timing.
