[RPG] How does damage from Arms of Hadar get assigned

From page 11 of the Monster Manual:

A creature that can make multiple attacks on its turn has the Multiattack ability.

So Multiattack is an action that allows a monster to make multiple attacks; that's the whole point of it.

If you look at any monster that has Multiattack, such as the Aboleth (first monster in the book with Multiattack) its attacks will be listed individually, and then Multiattack will say something like:

Multiattack. The aboleth makes three tentacle attacks.

So even without the general rule, the monster's statblock usually specifically states that Multiattack is made up of multiple attacks.

This being the case, the Rogue can only Uncanny Dodge against a single attack, but on the other hand, the creature has to make a Wisdom saving throw every time it wants to attack someone protected by Sanctuary.

Rules designer Jeremy Crawford addressed this question in a September 2016 tweet:

Magic missile. RAW: You roll 1 damage die (see "Damage Rolls," PH, 196). RAI: It doesn't matter; you choose.

Although Jeremy Crawford's tweets are no longer considered official rulings, the rules he refers to do answer the question. Per the "Damage Rolls" section (PHB, p. 196):

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, though the ruling in the tweet never really made it to the official Sage Advice Compendium, the "RAW" statement in the tweet accurately reflects the rules.

You are correct that you only need to roll once for all darts, regardless of the number of creatures hit, according to the Rules As Written. But tradition and fun (I like rolling dice!) can override this with little to no effect on the damage dealt, according to the Rules As Intended.