at ExpValue.set_int (ExpressionValue, set integer)
Photon.RoomPlayerCount (Count as in number, as in int)
str(13_room_names.SelectedText) (turns 13_room_names.SelectedText into a string, which it already is so using str() is unneccesary anyway)
I'm not 100% sure what you exactly wanted to do. It's either that you wanted to do
Photon.RoomPlayerCount(int(13_room_names.SelectedText))
Or, since you have room_names in there, you didn't want to set Photon.RoomPlayerCount but Photon.RoomName (or something along those lines. I'm not familiar with Photon)