The Rule of Three
Academy award-winning sound designer Walter Murch has discussed the phenomenon known as "the rule of three", which states that when the number of similar simultaneous events reaches three, an audience can no longer relate those sounds to specific on-screen actions.
Murch sites an example of adding footsteps in postproduction to match an actor walking on-camera. When done properly the audience relates those steps to the actor. The same is true when there are two actors and two sets of footsteps. However, when the number of people walking at the same time reaches three, the audience can no longer match the footsteps to a particular actor. The rule of three applies to any action, like gun shots, car-bys, explosions, and others.
There is evidence that for some time people have been aware of the rule of three. Consider the Chinese character for an individual tree and the Chinese character for a forest, where you can no longer separate the individual trees from the group. The forest character is actually three of the Chinese characters for an individual tree.
As Walter Murch was creating the sound design for Apocalypse Now, he discovered that because of the rule of three, his soundtrack needed to be simplified, so that the audience could more-easily relate the sounds to the action on screen.