You can easily take a map and just spawn additional players. But everything else is up to you. UE can’t decide what shall happen if two guys meet online. 
You can launch your single player game as server or listen server just by adding a command line argument to the executable. But I don’t think the netcode can handle thousands of clients straight out of the box.
Can you create a single player game and change it directly in an online game through blueprints without changing maps and cinematics etc,
And How can you create a server with ue4 that for example thousands of people could play on it around the world?
Thanks for the info
but what can i do if i for example release an online game for everybody and if for example 10 players in 1game, can other players start another game so that there are 100 peoples are playing multiple games without any problems?
There are no spesific limitations to the amout of players you can have on a server. It all comes down to how much trafic is going on and how optemized your network code is.
The more data thats beeing transfer the less players you can have online.
Of course when a new player joins more trafic will flow over the network.
So it all boils down to your network code.
I’m starting to get how servers works, thank you!