Who are the supporting cast?
When you write, unless your character is stranded on a desert island, there's going to be incidental folks milling around. You don't need to characterize them. But those certain few people who impact the hero and the story, the ones who need to be there for the story to move forward--or over hurdles--up mountains--or backwards, they must be characterized and they are your supporting cast.
They, but their very grouped presence, will tell us about your hero. Are they all nerds? Jocks? Lawyers? Misfits? It helps us categorize the hero. If he longs to be a caped crusader in his secret dreams, if he always gets the winning touchdown, if he is grounded in career, or if he is a rolling stone, the gang says something. Maybe he wants to break away from his gang...why? And why is that difficult?
Think for a minute how the above paragraph's final questions could grow and expand a story into more than the main plot.
Why are they here?
They better have a purpose. If it's the gal-pal the heroine has coffee with every Saturday morning and they just chat about things that have nothing to do with the plot...why I this scene in the book? Characterization? Hmmm. Maybe you need to know this about her. Your reader may not.
If they are just backup, how are the nerds going to fare in a fist-fight, and what might they do to prepare? If they are each a unique facet of a specialized team, they are stronger as a unit than as individuals and the misfits might have all found their place--and their leader.
Think for a minute how the above paragraph's obvious "stereotypical" examples imply subplots that could grow and expand a story into more than the main plot.
Those people, those extras, are just more opportunities for you to do your job as an author, to engage and entertain readers, and make them care about the characters and the plot and keep reading.
How many is too many?
There is no right or wrong answer. You can have as many as it takes to tell the story. If you can combine 'roles' great, if you can't don't STOP writing because you think you have too many! Often, solutions present themselves. If it is a real problem, an editor will let you know.
Sometimes, you may streamline things by making the hero's best friend have a career so he can be the someone who provides pivotal information, and is willing to give the info to our hero because they are pals. Or maybe it's the jerk who bullied him in school that he just saw at a high school reunion and learned that dude has access to a certain facility hero needs access too. *Not exactly part of the hero's supporting group, but plots are better when things are difficult for the hero. So getting info from the pal vs. sneaking the access ID from a former bully...which do YOU think is more engaging?
BAND: SMASHED GLADYS
SONG: CAST OF NASTIES