Hypnotic Pattern - Action Economy to Shake Awake

You create a twisting pattern of colors in a 30-foot Cube within range. The pattern appears for a moment and vanishes. Each creature in the area who can see the pattern must succeed on a Wisdom saving throw or have the Charmed condition for the duration. While Charmed, the creature has the Incapacitated condition and a Speed of 0.

The spell ends for an affected creature if it takes any damage or if someone else uses an action to shake the creature out of its stupor.

Shake isn't an action. If multiple creatures were affected by this spell, how many could be awakened in a single action and how close must the creature who is doing the shaking be?

It would be nice if the language was clearer and said something like, "someone else uses an action to shake a single creature out of its stupor". 

Why is this important? My DM interpreted the ambiguity to say another creature could reach out and shake multiple creatures within a single action. If I had 2 free hands and 2 creatures were within my reach, I could see that as possible, but what if my hands weren't free? It's a rabbit hole of debate in the middle of battle, bringing the battle to a halt while we debate the RAW. The DM wanting to wake multiple and the player wanting to limit to 1. 

My reading is that it is one Action for one creature. If you want to do multiple creatures in a turn you need something like Haste or Action Surge. Part of the reason I would say you can't do two at once is because you seem to need to be somewhat focused on the creature you are waking up since it is not a Bonus Action.

That is just my interpretation, however.

It says "The spell ends for an affected creature if it takes any damage or if someone else uses an action to shake the creature out of its stupor"  That is singular.  Explicitly one action to shake one creature.  This seems cut-and-dried to me.

If the DM wants to wake multiple creatures, assuming the waker has multiattack, he should use the attack action and deal damage to them =)

Quote from MrChen911 >>

Shake isn't an action.

Shake is defined as an action in the spell. Usually the fastest way to break a bunch of people out of hypnotic pattern is dispel or low levels of damage (for example, cast magic missile, one missile per target you want to wake up).