Here's how the designated-shooter policy limits the number of rounds fired. In the presence of the violent armed EDP, an armed domestic violence offender or by any armed criminal, each officer present would technically be able to easily articulate that they were justified to open fire because they felt an imminent threat.
Under a designated-shooter policy, a primary officer, secondary backup officer and backup team are assigned to perform specific duties to prevent every cop present from opening fire at the same subject unless it's absolutely necessary.
Instead of having a perpetrator get shot at with 50 or 60 rounds from the firearms of a dozen or more LEOs, the designated shooter would open fire along with the designated secondary or backup shooter also opening fire if the engaged subject continued to pose a threat after being shot at by the primary designated shooter.
If the primary and secondary shooters stop the threat, then everyone else involved in this enforcement action will stand down. Should the armed individual or violent subject continue to engage the police after being engaged by the primary and secondary designated shooters, then the designated backup team would be cleared to open fire.
When it comes to using authorized deadly force, a law enforcement agency can still look bad in the eyes of the community, even when multiple officers can articulate that they felt that their life was in danger and opened fire. Officers often will not realize that every other officer or agent present felt the same way and opened fire for the same reason.