When casting the main characters for a story having two main characters can be limiting. Having a cast of a thousand main characters can be daunting. Plot depends on interactions. With two main characters the interactions are limited. For the purpose here, we'll call these two hero and heroine, What you have to work with is - How does the hero interact with the heroine and how the heroine interacts with the hero.
Add a third and we'll call this the villain and the number of interactions increases. Hero with heroine. Heroine with hero. Hero with villain. Villain with hero. Heroine with villain. Villain with heroine. This can add a depth to the story. Now the character labeled as villain doesn't have to be a bad person. His or her interactions just have to act in a counter fashion to one of the main characters.
Then we get into those kind of stories where there are many main characters. Tolstoy does this in most of his novels. This can be confusing and also one of the characters can be lost for a time and when they re-enter the action this may seem forced.
There's nothing that says a writer must stick to two, three or hundreds of people in their stories. Plot is really based on how people interact with each other.