Reason number 1- Repelling blast states that when you hit with "Eldritch Blast". At higher levels, Eblast has multiple beams. But that doesn't change the spell itself from being anything other than "eldritch blast". The creature isn't being hit by 4 different instances of "Eldritch Blast", its being hit by one instance of the spell that happens to have multiple beams. So even though multiple beams hit, they are still only being hit by the spell "Eldritch Blast" once.
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 roll (and damage, if appropriate).
As Repelling Blast triggers on a hit with the cantrip (as the discussion went, specifically Eldritch Blast, since this was 2014 rules being discussed; however, this hasn't changed with any significance when still talking about 2024 Eldritch Blast and Repelling Blast), each hit of the EB will trigger Repelling Blast. It doesn't limit you to once per turn or once per spellcasting.
Reason number 2- Eldritch Blast has a range of 120 feet. Lets say you target a creature that's 120 feet away exactly. You roll to hit and all 4 succeed. If each individual blast would hit the creature at different times, then the first would hit and push it back 10 feet. Now the creature is 130 feet away and out of reach of Eldritch Blasts range. By the rules then, each subsequent ray should miss since its out of range. But you already rolled and hit with all 4. So with that logic, all 4 rays must hit at the same time. So even if you think each ray should have its own 10 feet worth of push, they all hit at the same time and wouldn't stack the amount of distance that the creature gets pushed back per the rules.
Under the circumstance that it's exactly at the edge of your range, this would be correct; however, any shorter range would allow multiple pushes. And once it's out of range, you can't target it with subsequent beams anyway, as we saw above, targeting is sequential and not simultaneous.
If you're going to necro a post, at least do it right.
Wrong. Sage advice tells us:
As Repelling Blast triggers on a hit with the cantrip (as the discussion went, specifically Eldritch Blast, since this was 2014 rules being discussed; however, this hasn't changed with any significance when still talking about 2024 Eldritch Blast and Repelling Blast), each hit of the EB will trigger Repelling Blast. It doesn't limit you to once per turn or once per spellcasting.
Under the circumstance that it's exactly at the edge of your range, this would be correct; however, any shorter range would allow multiple pushes. And once it's out of range, you can't target it with subsequent beams anyway, as we saw above, targeting is sequential and not simultaneous.
Nice try, but here we are 6+ years later with 4 more pages of commentary.