How many players may be placed on a level?

This question is so popular, but I want to get detailed response.

To create the MMO we need keep many players on one level (space). This is important moment. There are many factors, that affect performance. Such as count of players, capacity of network, power of server… But there are much ways to optimize this moment. The BigWorld server used cellApps to reduce the loading on server clusters.
But what about the Unreal Engine 4 server? Which optimization we can use for creating massive multiplayer?
Run a several levels on different applications? All players has tend to going to offline and again going to online. And loading on clusters is very various in this case.

My verdict: we must create the manager of array of UE4 servers, and this is hard work. And many people already making such server-managers, I’m sure. But current work with UE4 in progress. Planned any working in this direction?