I think this is a very complex issue.
There might be a way to fake this by grouping players together, and basically running multiple games on a single map, but only replicating game/player state between players that are in the same ‘group’, but you are still going to be pretty limited to a handful of games, with a few players per ‘server’.
I think the ‘correct’ way, but far more involved, would be to completely replace the networking/replication system on the client, so that rather than connecting to an actual UE4 server, it would connect to some other server software, which you would have to write from scratch (could feasibly implement that in Node.js or Go, or whatever language/library of your choice). That would potentially be much easier to deploy in a large scale on something like Amazon EC2.
Maybe someone else can think of a more elegant/simple solution, but those are the only options that spring to mind for me.