To be technical, I would say "you need to deal that much damage in a single turn".
The number of attacks is immaterial in a game where characters' turns are atomic. In reality, you might only have time for a single sniper attack, and as soon as you have made your shot you need to retreat to avoid detection.
In D&D time effectively freezes when it's your turn. All that matters is if the guard is still alive when your turn is over (and to be precise, that the guard is alive when his own initiative count comes around).
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Wrong. The turn isn't static. Time isn't frozen. That is why there is a "reaction" available so that you can answer someone else's action. Each multiple attack/spell is one attack. You can decide which one to parry/counter. So time isn't frozen and that is why you need to deal that much damage in one attack. Thus the "sneak" attack. Thus the fact that most rogues only have one attack. Thus the only one sneak per turn... (Note: I know, no action on the surpise round, but you can take a reaction at the end of your first turn. Meaning you might win the initiative but you won't be able to react until the surprising opponent acts).
Once the first attack is landed, nothing prevents the "guard" whatever it is to call in reinforcement. He might not be able to take a reaction, but after a hit, anyone will call in for help (and that is not forbidden by the rules, unless I missed something).
Consider that what the players can do to monsters, so can the "monsters" do to them.
So there is that stealthy wood elf 12th level fighter with enough rogue level to hide as a bonus action and the Kensai archetype. He gets to smash down a player. (Two handed sword +1, GWM, uses dex to hit and dmg because he's a Kensai and advantage because he's the first to act...) First attack 33, Second attack 23, Third attack 23. Action Surge fourth attack 23, fifth attack 23 and sixth attack 23. Total of 148 damage on average which is more than enough to kill a lot of players. Then he hides if the player is still alive otherwise, unnoticed, he goes on to the next player and the next and TPK...
As soon as the second attack lands, the player will want to yell for help. Would you deny him his call for help? Would you? If so, I urge you to reconsider your position. If you allow the player a call for help, then so should a "monster" be able to do the same.
On the other hand, if the player die on the first attack, the player won't be happy but he will abide by your ruling.






things sometimes happen. But if you had deny him his call for help, be prepare for a storm of protest (even if he died on the second attack) and a potential full quit at your game table.