Two people

Joel was becoming quite impatient. He put his hands on his hips and asked again, "If there's something in that book that caused us to switch bodies, then there has to be something in there that will cause us to swap back, doesn't there?"

"I tried! I looked!" Kelly said while still frantically searching the pages, "There doesn't seem to be any sort of reversal."

"Did you try just reading the original spell again? It swapped our bodies once; maybe it will do so again!"

"I tried that several times! It seems like a one shot deal. I think once two people swapped, they can never swap again!"