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Even wilder is that [[Doppelgang]] has a limit that you can't even define depending on the complexity of the targets. For example of you x=7 on 7 [[Colossal Dreadmaw]] or other vanilla stuff things work out just fine. But if you do it on 4 Dreadmaw and 2 [[Jodah the Unifier]] Arena blows up. If you do it on x=6 on 6 ping deserts, all is good, but if you do it on 3 deserts and 3 [[Spelunking]], or x=7 on 7 ping deserts it's lights out.
But wait there is more. If anything on board interacts with the stuff you are coping, good luck guessing the limits of things. X=7 on 7 Colossal Dreadmaw with a Soul Warden and a Heliod in play, game over.
Seriously, just play it safe and go for x=4. Otherwise good luck rolling the dice on what Arena can handle or not.
As you perform various game actions, the rules engine generates a wide variety of temporary objects to track different aspects of state - layered effects, potential mana you could generate, etc. With multiplicative effects, the counts of these can get quite high, forcing us to take down that game (lest that game take down others). This is something we've been spending a lot of time on tracking, understanding, and improving.