Technically speaking, every dojo has a Sensei, and that can be only one person, the other ones teaching if s/he is not in the dojo at the moment being more correctly instructors but not the Sensei.
How do you figure that? It just means "teacher", it's how Japanese schoolteachers are called. You surely don't think every elementary school can only have one sensei? This is getting a little too "Highlander" for me.