I guess you could have dancing minus one in which Morris sides could leave an empty space so that members of the public could join in. Somebody would have to shout out instructions ............. or is that Northwest?

I seem to have this folk (what else) memory of sessions with 2 or 3 instruments being available via the BBC. Since the whole point of sessions is that all sorts of people can join in, they are almost by definition "Music minus some number" :

Session = musicians - x