I was reading about the multiplayer framework that comes with unreal engine 5 and was wondering could someone use that framework in the development of a MMO?
Hey @Game_Student! Welcome to the forums!
Regarding your question, that kind of depends on your definition of an MMO. My first thought is World of Warcraft. For something MASSIVELY multiplayer like that you’d need dedicated servers running all sorts of nonsense. So not likely, at least not in blueprint. Something like Destiny or Diablo? Hmmmmmmm possibly, if you get creative.
Really I guess the worry here is scope. COULD you use the framework for an MMO? Yes. Would I suggest it? Probably not, you should probably utilize a specialist with lots of net coding and C++ experience. 
Thank you for the reply. I am thinking about a mmo like Workd of Warcraft just wasn’t sure which direction to go.
Unreal’s multiplayer framework only provides game server and basic online services, such as account login, cloud save, etc - which is all non-MMOs need.
An MMO requires game servers, which Unreal provides, but also requires a lot of other services. No game engine can provide the services you need for an MMO as the required services are not part of the game, it is part of a sophisticated cloud backend.
The player, the game server and the backend all communicate with each other.