if (count <= 1) {
   ...
}

OnMessage(0x219, "MsgMonitor")
     MsgMonitor(wParam, lParam, msg)
     {

        SysGet, MonitorCount, MonitorCount
        SysGet, MonitorPrimary, MonitorPrimary

        count := 0
        Loop, %MonitorCount%
        {
            count++
        }

        IfLessOrEqual, count, 1
            Run, powercfg/ -setdcvalueindex SCHEME_BALANCED 5ca83367-6e45-459f-a27b-476b1d01c936 1 /* no external monitor power setting */
        Else
            Run, powercfg/ -setdcvalueindex SCHEME_BALANCED 5ca83367-6e45-459f-a27b-476b1d01c936 0 /* external monitor connected */
     }