If everyone at the Boulder store had been carrying a gun, how many would have been killed or wounded?

Your answer to gun violence to to have more guns.  Admit it.  You have a fantasy of being the heroic guy who takes down a shooter.  Get off it.  Ask instead, why is that a turd can walk into a gun store and buy a military weapon that can take someone's arm off with one round.  Shot in the neck, a person's head falls off.  Is that your dream?  To be seen as the cowboy who saved the whole store?  C'mom, Alex.  Boys stopped playing gun fight games as children.

Many, many more. Everyone who fired a gun, who was even seen holding a gun, would be thought to be a terrorist by everyone else, including by the police.

Not as many as were killed.

Probably two.  The first victim, and the murderer.  

The mythical law abiding gun owner that saves the day never seems to show up during these mass shootings