Please keep in mind that the MMO Kit is just a couple of tools for UE4 to make it more MMO-capable and basically a template project. It’s not a standalone server solution or anything like that. So the question is simply how many players can UE4 handle.
It all depends on the amount of game logic that the server has to handle and the amount of replicated information. I can’t tell you what’s the maximum that UE4 can handle for your game. If I had to guess based on your specifications (and I don’t even know if it’s point-n-click movement or WASD, if you can move camera beyond your character or if it’s always centered on your character, etc), I’d say 200 players for a single server is the maximum in your case if we’re being optimistic. If you place objectives around the pvp zone in different places, you can run parts of a level on one server, parts on another. If it’s necessary, on 3 servers, to reach your desired player limit in the pvp zone. They’ll never see each other at the same time, but they’ll all participate in one event. If the transfer between servers is done seamlessly without loading screen, they’ll probably never suspect a thing. And just generally speaking, your world should be divided into many areas, each area being run on a separate server. All MMOs do this.