So, the spell hits 5 creatures and they all fail the Str check. Do you roll 2d6 for each creature that the arms hit or just roll once for the damage that affects each of the 5?
It's one damage roll, it's applied to all characters.
How I do this in my games is have the damage rolled before anyone makes their save. So the caster determines the damage before the saves are rolled.
Then each character makes their save and the effects are adjudicated after the saving throws are made.
Page 196 of the PHB has this rule:
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....