The number of room members present in the room is the room property of name memberCount, and its value is available by StrixNetwork.instance.room.GetMemberCount(). You can check the value in the Update callback, for example, and invoke a particular method when it reaches the predefined number.
Or, you can use the RoomJoinNotified event. It is a room event provided by StrixNetwork.instance.roomSession.roomClient, which fires when a new member joins a room. You can subscribe the event to call GetMemberCount() described above in the handler and examine the number of members.