There is ControlPlane, but I didn't have much luck with it on El Captain.
Other option is running a never-ending AppleScript which calls your script on change of number of monitors. See this answer.
Trigger action on external monitor plug/unplug
There is ControlPlane, but I didn't have much luck with it on El Captain.
Other option is running a never-ending AppleScript which calls your script on change of number of monitors. See this answer.