The listroom expressions are used in an "On room list" trigger event.
To get this event to trigger, you'll need to use the "Request room list" action.
The index will specify which room's information you want from that list, as the list can contain multiple rooms.
To get the index number of your current room, you're going to loop through the list until you find the index of the room that matches "CurrentRoom". It will look something like:
On room list
Repeat Multiplayer.ListRoomCount times
Compare two values "Multiplayer.ListRoomName(loopindex)" = "Multiplayer.CurrentRoom"
Set text to "Multiplayer.ListRoomMaxPeerCount(loopindex)"
Stop loop