photon

// #Critical: we failed to join a random room, maybe none exists or they are all full. No worries, we create a new room.
PhotonNetwork.CreateRoom(null, new RoomOptions { MaxPlayers = maxPlayersPerRoom });

/// <summary>
/// The maximum number of players per room. When a room is full, it can't be joined by new players, and so new room will be created.
/// </summary>
[Tooltip("The maximum number of players per room. When a room is full, it can't be joined by new players, and so new room will be created")]
[SerializeField]
private byte maxPlayersPerRoom = 4;

public override void OnJoinRandomFailed(short returnCode, string message)
{
    Debug.Log("PUN Basics Tutorial/Launcher:OnJoinRandomFailed() was called by PUN. No random room available, so we create one.\nCalling: PhotonNetwork.CreateRoom");

    // #Critical: we failed to join a random room, maybe none exists or they are all full. No worries, we create a new room.
    PhotonNetwork.CreateRoom(null, new RoomOptions());
}

public override void OnJoinedRoom()
{
    Debug.Log("PUN Basics Tutorial/Launcher: OnJoinedRoom() called by PUN. Now this client is in a room.");
}

public class Launcher : MonoBehaviourPunCallbacks
{

// #Critical: The first we try to do is to join a potential existing room. If there is, good, else, we'll be called back with OnJoinRandomFailed()
PhotonNetwork.JoinRandomRoom();