I think many fictional films have 5 team members because that number allows for different skills and personalities in the group. For example in Battle Of The Planets, each team member has a different personality and approach to the situation to the solution that they would work towards. Jason, in Battle Of The Planets, has a anger issue and goes with the first idea to the solution, but the leader takes stuff calm and comes up with multiple ideas, and even when that other kid that was made in a lab he just comes up with the first possible idea in his head even if it is not good.
Another, example would be The Incredibles. They have a team of 5 people, but none of them are the same personality. For example, Mr. Incredible likes to play it quick, while Elastic Girl thinks her ideas out. Then the kids just react as quick as they can but still think it out. Lastly, another example would be The Avengers because Hulk has anger problems and just fights, but doesn’t think at all, while Iron Man plans out what he is going to do in detail.
So I think that the reason of have teams of 5 is that they would have a variety of personalities.