"A maze that can be built from popup barriers* and teleporters, splits/sorts groups of players to "calculate" the number of players who can't solve it further, the maximum reached stage represents the number of isolated players (stage 1 is when no one has entered the maze), the feedback version can also count players if every isolated** player is given an seperate way out.

It is also possible to use maze objects instead of popups to build the AND switches, after all they fit the name much better, but this will turn the maze into single use.

* the barriers in the drawing are supposed to be upside down, with the trigger at road height.
** this can be tricky, because you have to set up popups or maze objects to respond if only a single player is around."