Answer:
It means that if too many people try to do the same thing, they end up ruining it. If you have too many cooks trying to cook one broth or soup, they get in each other's way and end up adding the wrong things.
It means that a simple thing like a broth (or a movie script) is better when it doesn't have many authors contributing to it.
If more than one cook handles one pot of soup the ingredients like salt, for example, may be doubled or tripled (according to how many cooks handle the pot) thus spoiling it.