There is much to be gained by populating a story with interesting personalities, but personalities are not necessarily functioning characters. You can have as many “window dressing” characters as you want. Make sure, however, that each of the eight archetypes is represented by one of your characters.
For a given character, why would you pick one archetypal function over another? Simple: the archetypal functions are essentially descriptions of different personality types. Take the Sidekick archetype, for example. The Sidekick is described as a “faithful supporter.” If you select a character as the Sidekick, you have already said a lot about the kind of person it will be.
Note that the archetypal description says nothing about in what the character has faith or what it supports. This is why Toto in The Wizard of Oz can be a sidekick, but so can Renfield in Dracula. The Sidekick is not necessarily the faithful supporter of the Protagonist, but simply fulfills the dramatic function of illustrating how the qualities of faith and support fare in regard to solving the story’s central problem.
So, in choosing which archetypes you want to assign to which characters, select the matches in which the characters function best reflects its personality, and vice versa.
Dramatica Story Development Software