How do you go about making a mass multiplayer?

Out of the box, UE4 seems like a good local multiplayer solution but when one talks about mass multiplayer 400+ players then users seem to say that there are problems involved. Now, even with the (i think 64 player limit) what is a rundown procedure for setting up multiplayer features, Accounts, Individual Inventory and Settings, last Positions for evey player, Achievemtns, And Mobs that appear same for every player.