How many times do you roll damage for Scorching Ray?

Question

The rules for damage rolls say:

If a spell or other effect deals damage to more than one target at the same time, roll the damage once for all of them.

So for the spell magic missile, it seems quite clear that you only roll the damage once. Magic missile says:

The darts all strike simultaneously.

So you roll the damage once for all the missiles, rather than rolling the damage once for each missile.

Notably, magic missile does not require any attack rolls - the missiles just hit. Additionally, magic missile explicitly states that the missiles hit at the same time.

These two facts make magic missile seem quite different from scorching ray. Scorching ray calls for an attack for each ray and does not have a statement concerning the simultaneity of each hit:

You create three rays of fire and hurl them at targets within range. You can hurl them at one target or several.

Make a ranged spell attack for each ray. On a hit, the target takes 2d6 fire damage.

Further, the rules for attack rolls seem to suggest that when you make an attack roll, you roll damage for that roll. From the rules for making an attack:

You make the attack roll. On a hit, you roll damage, unless the particular attack has rules that specify otherwise.

How many times do you roll damage for scorching ray - once for each individual hit, or once for all the hits?

Note, this situation is different from the one described in this question: How many damage rolls do you make for the Prismatic Spray spell? Prismatic spray uses saving throws, not attack rolls.

Answer 1

You make one damage roll for each ray that hits.

The rules for making an attack tell us:

Whether you're striking with a melee weapon, firing a weapon at range, or making an attack roll as part of a spell, an attack has a simple structure.

1. Choose a target. [...]
2. Determine modifiers. [...]
3. Resolve the attack. You make the attack roll. On a hit, you roll damage, [...]

Spells like Scorching Ray, Eldritch Blast, and Steel Wind Strike have the caster make multiple attack and damage rolls, and selecting the targets sequentially.

Magic Missile is the edge case here. Because the darts of force strike simultaneously, you make a single damage roll.

Answer 2

Per the Sage Advice Compendium, you roll separately for each ray

The Sage Advice Compendium includes the following three questions and answers relating to the scorching ray spell. The first two appear in the sorcerer section:

Q. Elemental Affinity improves one damage roll of a spell, not multiple rolls? So with scorching ray, I don’t add my Charisma modifier to each ray that hits?

A. That’s correct. Elemental Affinity benefits one damage roll per casting of a spell, even if the spell allows more than one roll. So, for example, the feature improves one of the rays of a scorching ray spell or one of the beams of an eldritch blast spell.

Q. Can Empowered Spell affect all the rays of a scorching ray spell, or just one?

A. A sorcerer’s Empowered Spell could affect more than one ray of a scorching ray, abiding by the feature’s die limit. For instance, if you create three rays with the spell and you have a +3 Charisma modifier, you could reroll one of the damage dice for each ray, or two of the damage dice for one ray and one of the damage dice for another one.

The last such ruling is in the section on spell attacks (emphasis mine):

Q. When casting a spell that affects multiple targets, such as scorching ray or eldritch blast, do I fire one ray or beam, determine the result, and fire again? Or do I have to choose all the targets before making any attack rolls?

A. Even though the duration of each of these spells is instantaneous, you choose the targets and resolve the attacks consecutively, not all at once. If you want, you can declare all your targets before making any attacks, but you would still roll separately for each attack (and damage, if appropriate).

All of these quotes establish that scorching ray involves multiple damage rolls (a separate roll for each ray that hits).

The rules themselves also lead to this conclusion, as detailed in BlueMoon93's answer.