In every story, there’s a hero. You should always have one hero because this is the person who pursues a goal and either achieves it or fails to achieve it. Along the way, the audience roots for the hero to succeed.
Besides the hero, there are three other major characters: the villain, the mentor, and the ally.
The villain opposes the hero and forces the hero to change. The mentor, on the other hand, helps the hero and also helps the hero to change. The ally physically helps the hero but also exists so the hero can help the ally change into a better person. Then the ally turns around and helps the hero in the end.
In many stories, there’s only one villain, one mentor, and one ally. In “Star Wars,” the main villain is Darth Vader, the mentor is Obi-wan, and the ally is Hans Solo. Throughout the story, “Star Wars” has the hero (Luke) interact with Darth Vader, Obi-wan, and Hans.
However, in some cases you may have multiple villains, multiple mentors, or multiple allies. This might be necessary when having a single villain, mentor, or ally doesn’t make sense.
For example in “Little Miss Sunshine,” the hero’s goal is to get to the beauty pageant and compete in it. It doesn’t make sense to have a single, Darth Vader-like villain, constantly trying to stop her. Instead, it makes more sense to have multiple, unrelated villains try to stop her in different ways. This makes the story more believable than having a single, evil villain constantly trying to stop the hero like the coyote in a Road Runner cartoon. When you have multiple villains, they may all be different, but their goal is the same: stop the hero from reaching a goal.
Other times you may have multiple mentors. In “Legally Blonde,” the main mentor is a lawyer who’s helping the law professors. This lawyer gives the hero advice to pass her classes and also sponsors her in the end so she can defend her first court case.
However, when the hero’s law professor makes a pass at her and she turns him down, she runs to the beauty parlor for comfort. It doesn’t make sense for the mentor to be in the beauty parlor so a substitute mentor appears there instead. In this case, it’s a female law professor who encourages the hero to stay in law school.
This female law professor plays the role of the mentor in that one scene because it makes sense that a woman would be in the beauty parlor, not the mentor who’s a male lawyer.
Finally, there may be multiple allies. In “WALL-E,” one ally is the human couple looking for love. Another is the starship captain looking for meaning in his life. The third ally is the army of rogue robots who are rebelling against the villain.
All of these allies help the hero in different ways. The human couple helps save a bunch of babies in the end. The starship captain helps turn off the villain in the end. The rogue robots help the hero battle the villain’s security robots in the end.
Because each ally helps the hero in a different way, it doesn’t make sense to have just a single ally to help the hero like in “Star Wars” with Hans Solo.
When writing your own screenplay, play around with the idea of a single villain, mentor, and ally, or multiple villains, mentors, and allies. Generally it’s easier to have a single character for each role but depending on the story, you might need multiple characters to play a villain, mentor, or ally.
The key is to make it logical. The fewer characters, the easier your story will be to understand so only use multiple characters in a particular role when necessary.