Rounded characters are people, flattened on the page. Flat characters are just things used to add plot points or to allow the main characters to be more rounded. They may think they're the main characters, but for the story they are only facilitators.
Example:
Hamlet is a rounded character. He has his motivations, his strengths and his flaws. He seems like a real person, albeit larger than life and a little over the top, but a relatable character
Rosencratz and Gildenstern are flat characters. They pretty much just appear to allow definition for the main characters.
There can be as many rounded characters as you feel comfortable. Hamlet has Claudius, Gertrude, Pollonius, Ofelia, Hamlet, Laertes, Horatio and plenty more. They all think they are the main characters, they all think the story's about them.