To extend the scale and scope of standard UE Dedicated servers, you’ll need external back end support. There’s several projects out there to assist with massive multiplayer in Unreal, but one that I’ve enjoyed following is the Open World Server project: http://www.sabredartstudios.com/
By using an API and several support servers, the scale of server instances is only limited by budget… of course changing the UE server source to increase the number of players per server is an entirely different beast.