I use 1 or 2 main characters for short stories (usually 1), and 1 to 4 supporting characters. I use 1 or 2 main characters for novellas, and 3-6 supporting characters. Thus far, I've used 3-5 main characters for novels with about 3-20 supporting characters.
There's usually a few mentions of other people with little description and no background, such as portly cook, tall shopkeeper, man on the street #2 . . .