Is the Hunter ranger's Volley feature considered a single attack or multiple attacks?

Question

The Hunter ranger's 11th-level Multiattack feature has two options. One of them is 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.

My original interpretation is that you make a large number of single attacks as a result of your action, but the I read this section of the "Sage Advice Compendium":

No. Whirlwind Attack is unusual, in that it’s a single attack with multiple attack rolls. In most other instances, an attack has one attack roll.

The description of Whirlwind Attack states:

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.

Since Whirlwind Attack is worded almost identically to Volley, is Volley considered a single attack?

Answer 1

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):

Multiattack

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.)

Answer 2

It depends on with which rules it is interacting

The Volley feature is an Action that produces a single attack

My apologies for being pedantic here, but it is actually relevant to this question and its duplicate.

The question is "Is the Hunter ranger's Volley feature considered a single attack or multiple attacks?" If the Volley feature was, itself, an attack, then a Ranger with Extra Attack could take the Attack action, permitting them to make two attacks, and then with one (or both) of those attacks, use the Volley feature. That can't happen, though.

Instead, the Volley feature permits the Ranger to take a Volley Action, and the Volley Action permits a Volley attack. The Volley attack is a special kind of attack that may have multiple attack rolls associated with it. The Volley Action cannot be taken in any combination with the Attack action (unless the character is permitted multiple Actions on their turn).

The Volley attack is (usually) a single attack with potentially multiple attack rolls

This part is explained well by @Rykara's answer to this question. The Sage Advice Compendium says that Whirlwind is a single attack with multiple attack rolls¹. By analogy, its companion feature Volley should work the same way. That is normally enough to explain most rule interactions. For example, Would the Slow spell block a Hunter ranger from using Volley or Whirlwind Attack?² No, because all the attack rolls from Volley are generated by a single attack.

Sometimes the Volley Attack is treated as multiple attacks, at least in accepted answers on this site

The general rules as written assume that any given attack produces a single attack roll against a single target, and typically this is true. When it is not true, however, it becomes difficult to interpret rules that assume "one attack = one roll, on one target". For example, this question asks about the interaction between Volley and Horde Breaker^3.

Horde Breaker says:

Once on each of your turns when you make a weapon attack, you can make another attack with the same weapon against a different creature that is within 5 feet of the original target and within range of your weapon.

In particular, the question asks whether Horde Breaker can be used against one of the targets of the Volley. There is currently just one answer to this question, and it is both highly upvoted and accepted. It says "There is nothing that prohibits this interaction. The ranged attacks made from Volley count as weapon attacks [sic], so any of them can be used as the "trigger" for Horde Breaker." But note this logic doesn't work with the ruling supplied in SAC. If the Volley is one attack with multiple attack rolls, then there are not weapon attacks, there is just one weapon attack. And each of the targets of the Volley are the original target, so it is not possible to target any of them again with Horde Breaker⁴. Either the accepted answer to the question is wrong because of the SAC ruling, or for some reason the SAC ruling does not apply in this case and the Volley produces multiple weapon attacks. (I prefer the former).

Similarly, this question asks about how "Hunter Abilities", including Foe Slayer, interact with Extra Attack. There is a single answer there with a positive total. It is highly upvoted and accepted. It says that you can use the bonus provided by Foe Slayer "on one of the Volley attacks". Here again, if the SAC ruling is actually followed, there is only one Volley attack, and thus you should be permitted to use the bonus provided by Foe Slayer on all the attack rolls (or damage rolls) which proceed from this single attack. Yet the accepted answer argues that you cannot do this. The only opposition comes from a comment mentioning that the poster doesn't agree with the SAC ruling anyway.

Sometimes the Volley attacks are treated as multiple attacks, but because of more specific rules interactions that are explicit

Suppose you make a Volley attack. How many pieces of ammunition do you use? The ammunition property says:

Each time you attack with the weapon, you expend one piece of ammunition.

It appears that even though I am making multiple attack rolls, since I am making only one attack with Volley, I need expend only one piece of ammunition! Apparently the same arrow is making each different attack roll! This is even better when I consider special ammunition. Suppose I shoot an arrow+2. I get to add the +2 to hit and damage to each of the targets of my Volley, because that one arrow is making all of the attack rolls.

But wait. In this case, Volley itself says:

You must have ammunition for each target, as normal, and you make a separate attack roll for each target.

Now, we can argue about what "as normal" means. It could mean 'You must have just one arrow for multiple targets, as normal when you are making a single attack with ammunition'. So, cool - now I need to Volley with an arrow of slaying⁵! But more likely "as normal" means 'You must have one arrow for each target, as normal when firing a ranged weapon'. This certainly would be the ruling with more verisimilitude. Here, even I, a firm supporter of 'The SAC ruling says it is just one attack', agree that you need one arrow for each target of the Volley, justified by what the text of Volley itself says. So in the case of ammunition expenditure, we can treat Volley as if it was multiple attacks, and not one attack with multiple rolls.

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¹ Sage Advice notwithstanding, it is more correct to say Whirlwind and Volley are single attacks with potentially multiple attack rolls. They target "any number of creatures" but those numbers could include 1 or even 0, resulting in one or no attack rolls.

² Volley is also discussed in: Does the slow effect affect all attack types?.

³ This question asks the same thing about Whirlwind and Horde breaker, and the accepted answer uses similar logic to come to the same, possibly erroneous, conclusion.

⁴ One could argue that among the targets of Volley there is a first target, in the sense that among the attack rolls one must be rolled first. However, all of the targets of Volley are selected at the same time, with the election of "any number of creatures within 10 feet of a point", so all of the targets are "original" targets, and thus not eligible for Horde Breaker. There could be a novel target which was within five feet of one of the original targets of Volley but which was too far from the point chosen for Volley itself to count. This, more distant, target would be a legitimate target for Horde Breaker. Or, the ranger selecting targets for Volley could deliberately not include a legitimate target with the Volley itself, but then add that target in with Horde Breaker. However, there is no net gain to such a play.

5 The arrow of slaying description says that after it damages a single target it becomes a normal arrow, so I won't be able to use the slaying effect against each of the targets of my volley. But it would be a great bet-hedging effect, since I could miss multiple targets and still apply the slaying effect to the Volley target I managed to hit.