Hi Tienbao,
To get number of users present in a room, call getLiveRoomInfo API and you will get the response in the onGetLiveRoomInfoDone listner method of RoomRequestListner.
On the other hand there is a work around that you can use an alternative, as the room is created by player A, so by default he will be the first user of the room to join. You can maintain a counter and assign it by 1 when A joins the room. Later on keep increasing that counter by 1 when you get onUserJoinedRoom notification callback. So that way when B joins counter will be 2 and when C joins it will be 3 and so on.
Let me know if it helps.