Construct 2 shows tips on parameters when edited.
For "RoomPlayerCount" it should be like "Count of player currently in room." and "Room name." for it's "Name" parameter (all tips are in edittime.js of plugin). You need put name of room which count of player you are interested in.
Note that this method make sense only when client is in lobby. For count of players in currently joined room, use ActorCount.