"Beginning at 3rd level, once per day an arcane trickster can declare one melee or ranged attack she makes to be a sneak attack (the target can be no more than 30 feet distant if the impromptu sneak attack is a ranged attack). The target of an impromptu sneak attack loses any Dexterity bonus to AC, but only against that attack."
Do you only get one hit, or can you do an full attack's worth of sneaks when you use this?
It is one attack/hit, not all of the attacks.