It all depends on when the monsters becomes aware of the adventurers, either through the monsters seeing (or in some way perceiving) the adventurers, or when the adventurers attack.
Lets say the heroes move out of hiding. If the monsters see the heroes appear from behind the cliff, this is the surprise round. Each hero acts and then the surprise round is over.
If the heroes manage to sneak up to the monsters, (or are not seen moving out from behind the cliff) then the surprise round would likely start with the first attack. So if all the adventurers managed to sneak up, they might all get to attack before the monsters get to react (their reaction coming in the round after the surprise attack).