Hints for developing semi mmo sandbox

I am going to create a multiplayer game with semi mmo style, when many peoples can play togehter.
I know UE4 supports multiplayer gameplay but it is capable of 100-200 players on server or i need to write own networking code?
Another question is, UE4 is now capable of creating multiplayer sandbox game? I can achive that with c++ scripting or i should go for source code?
I am at very begining of creating concept and i looking for key technologies and solutions for my game. Every key word for my serach will be great. :slight_smile: