Definitely no guarantee! I had originally booked for our party of 4, but then my oldest decided she didn’t want to go. After we had been seated for a while, it looked as though our party of three was not going to have anyone joining us. Last minute though, we had a solo traveler seated in the available chair. We had no issue with it though, and enjoyed chatting with him and giving him tips for his upcoming WDW trip.
Oh, he also mentioned that he had booked the one seat the day before
It’s likely not that common for a solo park visitor to book the experiences, but it can happen!