Multiplayer Online quiz game in Unity (Only Backend)

Need to develop an online multiplayer 1v1 or 1vN game where connection is established between players and score is transmitted between them. At the end of the time the scores should be synched and winner should be declared.

Frontend is already developed in Unity. Need to develop the backend and multiplayer layer.

Needs to be highly scalable and support upto 10000 concurrent users. You can suggest your own preferred technology for networking.