To be blunt, MMOs are usually terrible project ideas… it’s the perfect way to never finish.
So my suggestion is just get it working assuming one server for now. That way if you need to scale back your plans, you haven’t invested time for nothing. All the work you are doing to get it working assuming you will use a single server will be required anyways if you attempt multiple servers (which I’m pretty sure UE4 has absolutely no support for.)