Sage Advice sets the precedent that Volley is one attack with multiple rolls
At level 11, a Hunter ranger gets a "Multiattack" feature per the PHB. That multiattack manifests in one of two ways: Either a ranged type (Volley) or a melee type (Whirlwind Attack):
At 11th level, you gain one of the following features of your choice.
Volley. You can use your action to make a ranged attack against any number of creatures within 10 feet of a point you can see within your weapon's range. You must have ammunition for each target, as normal, and you make a separate attack roll for each target [...]
Whirlwind Attack. You can use your action to make a melee attack against any number of creatures within 5 feet of you. with a separate attack roll for each target [...]
In essence, a Hunter ranger gets the Multiattack ability but can tailor it to match the melee or ranged combat style they have chosen to develop. The wording of the rules are identical other than the ranged/melee qualifications.
Both are unclear as to whether the ranged/melee "attack against any number of creatures" is a singular attack (which targets multiple creatures), or multiple attacks against each of the creatures.
The Sage Advice Compendium specifically allows only the former interpretation for the Whirlwind Attack feature:
Whirlwind Attack is unusual, in that it’s a single attack with multiple attack rolls.
Because there is common wording between the two variations of the Multiattack feature and because the Sage Advice Compendium informs us that one of these two variants is one attack with multiple rolls, the other (Volley) must necessarily be treated the same way.
Lead rules designer Jeremy Crawford tweeted in September 2014 that this interpretation matches designer intent:
Like Whirlwind Attack, Volley is a single attack with multiple attack rolls.
(Hat tip Speedkat & Rubiksmoose.)