Please forgive me if this question seems a little bit weird. I'm mixing a feature which has a tunnel scene where police and bad guys are shooting each other. The director added many CGI gun fire on screen so that it looks like everyone is opening fire at the same time! So if I add gunshot sfx on every firing CG, the whole thing will sound like a mess! Does anyone have experience of this kind sound design? What guidelines should I follow?
Approach the scene how you would see it. It's likely that all of the guns weren't fired in the same millisecond, plus some guns are louder than others or have different firing patterns that would probably mask other sounds.
There's also other elements to a scene like bullets hitting bad guys, ricochets, sirens, explosions, yelling -- many ways to provide an audio landscape by choosing the proper arrangement of FX as you would perceive them.
That reminds me of the Lord Of The Rings documentary bonus material (almost as good as the movie itself!), in which a sound engineer described the battle with the Oliphants. They first included each and every stamping sound of the oliphants and it was constant thunder. So the only solution was to remove most of the stomping to emphasize it.
And here is my point: The final scene sounds really impressive, although most of the visible stomping makes no sound. As in your scene, there is a lot of noise, even arrows being released and hitting something. The sounds belong to the things the eye is guided towards. So instead of adding all guns, then all cars etc., start with the most obvious items and descend until you reach background noise.
The audience watches the scene only once, and before they notice that one pistol in the corner didn't make a sound, it is already over. Another nice example I think is the famous Matrix Lobby Shoot-Out.