As is usually the case, the Dungeon World mechanics allow for a wide variety of narrative freedom, but that same freedom can make it tough for new players (and gamemasters) to establish reasonable expectations for what they should be able to do. My approach in Dungeon World is to focus more on examples of how a thing would work rather than new rules (per se) covering situations.

Anyway, here’s what I told them about how to handle the Hack and Slash move if you want to damage multiple targets with one attack.

“Neirin attacks all eight kobolds with his knife. Hack and Slash?”
“Daggers have the range tag of ‘hand’. Neirin can attack the four kobolds who’re are right on top of her, but it’s risky. If you succeed, your damage applies to all four. And you don’t have the option to do extra damage.”

“Carys attacks the two brutes with a mighty swipe with his axe, as both have engaged me in melee at close range.”
“Hack and Slash. On a partial, you only hit one.”

 “Heulwen attacks the horde of goblins with her halberd. How many have engaged me in melee?”
“Three have swarmed you at ‘hand’ range, six at ‘close’, and four more are gingerly picking at you with spears at ‘reach’. You’ve got a shot at the six that are close, but it’s risky because of the three goblins grappling with you, and even more risky because of the four with reach. And it’s going to take a minute.”
“I go for it!”
“Roll the first Defy Danger for the goblins at hand”.
“Neirin, you see Heulwen swamped with goblins. What do you do?”
[Action]
“Heulwen, roll to Defy Danger from the goblins with reach.”
“Carys, you see Heulwen swamped with goblins, and the Neirin is picking them off from range. What do you do?”
[Action]
“Heulwen, it took you a minute, but you make a sweeping attack against the close goblins. Make a Hack and Slash Move against multiple targets.”