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?