If HS hits one target or many doesn't matter for total damage done because the damage splits evenly. However if HS only hits the closest target then that's a good thing as I gives us some control over who it will hit.
It hits 15 times - and it hits the closest target as well.
Should that target die, then it finishes off on the next closest target. You can test this easily yourself by killing critters, or sha-lings in the vale.