Well, first up, we need to see what we're dealing with.
If you take any damage while
you have 0 hit points, you suffer a death saving throw
Ok. Now let's look at Scorching Ray for an example.
Make a ranged spell attack for each ray. On a hit, the target takes 2d6 fire damage.
You suffer a death saving throw failure when you take damage, and you take damage each time a ray of Scorching Ray hits you. So if you're hit by 3 rays, you take 3 death saving throw failures. Eldritch Blast uses the same wording.
Now, Extra Attack. Extra Attack says:
Beginning at 5th level, you can attack twice, instead of
once, whenever you take the Attack action on your turn.
So you can make two attacks, and that's all it means. When you make an attack:
- Choose a target. [...]
- Determine modifiers. [...]
- Resolve the attack. You make the attack roll. On a hit,
you roll damage, unless the particular attack has rules
that specify otherwise. Some attacks cause special
effects in addition to or instead of damage.
Each time you make an attack, you roll damage. You don't, for example, make all of your attacks and then roll damage for them all cumulatively. If you use Extra Attack to hit an unconscious creature twice, it takes damage twice, and suffers 2 death saving throw failures.
As additional backup (thanks Adeptus), we have:
If the damage is from a critical hit, you suffer two
A critical hit is something that happens to an individual attack, like one ray of Scorching Ray or one attack from Extra Attack, not to the whole of either of them.
As far as brutality is concerned, I would point out that there are good reasons not to attack unconscious creatures: Unconscious creatures aren't a threat. From the perspective of a monster, it would generally make more sense to deal with the adventurers who are still trying to kill them than wasting time finishing off the ones who are already out of the fight.