you can duplicate the tab to simulate more players although it isn't a perfect representation because scratch's cloud servers will lock you out temporarily if one account sends too many changes to the cloud variables per second, so having multiple tabs may make a few tabs get locked out temporarily. this multiplayer engine should theoretically support infinite players although I highly doubt that would work because it would get extremely hard to play because of the lag, it also only uses 1 cloud variable, the reason for this is because the more cloud variables you add to a game, the slower the cloud variables respond, so I think this method should be faster than games that use multiple cloud variables, but to be sure I will do more testing.
