Depends on a LOT of different things (what skill you're using, what monster you're fighting, gear, etc etc).
It's not really about the damage. It's about efficiency. How fast can you kill the mob? How quickly do you gain EXP? How is your survivability?
For example, if you can Tank 10 monsters and AOE them without a healer, the monster you're fighting is probably too weak.
But on the flip side, I also feel like AOEing a mob of less than 4 targets with a healer is a waste of time. (Even 4 is questionable to me, I like to do 5 or more if possible).
I think some more pertinent questions to ask are
1) "How many monsters should I pull for 1 mob?"
2) "How many levels over me should I attempt to AOE?"
It's hard to straight up answer the first question, I think it comes from experience as you get a feel for how strong your character is and how capable your healer is. When pulling a lot of monsters, it gets to a certain point that even if they aren't doing much damage, their flying crits will interrupt your skill casting and you won't be able to take them.
The second question is pretty much gear dependent. If you're a low level Assist (->Billposter) with store-bought junk gear, you can still AOE monsters in Saint Morning; even if they are 1 or 2 levels under you, you can still pull a decent size mob and gain decent EXP. In the later levels you will usually be fighting monsters that are a higher level than you.
Due to game updates the Stamina stat is being questioned. It becomes possible to AOE with little or no Stamina if you have very strong equipment (very highly upgraded, like +8 set) and continuously spam cash shop items (Balloons + "FOR"). Awakened stuff can help too. If you have the $$$ for this stuff, the points you save from STA can be used for your main damaging stat (STR/DEX/INT) instead which will help you to kill faster. In the past, 1:1 stat builds were the norm for AOEing, but now this is no longer necessary.