How Many Characters Should I Have In My Book?

How many characters does it take to make a good book?

I've read a few books and I've noticed there is the protagonist, the antagonist, a supporting character or two, and minor characters that reveal the plot.  So it might depend on how many plot lines, how many problems or how much trouble you want your main character to get into.  So no less than four?

I've also seen a protagonist be the antagonist fighting the trouble in his own head.  That would be one.

But whether you have one or twenty characters they all have to have a reason to be there.  But how do you know if they are need to be there?  I like asking my characters "If you were never born, how would the story change?"  The ones that cough or stutter are out.  If they don't have an immediate response, except for those characters that stutter as a regular means of communication, they're slashed.

I find this is also a good exercise to take out a character to see how the story flows.  It strengthens my main character as he has to "find another way" out of the labyrinth.  Sometimes I come up with good stuff.  Other times I see a weakness.  On rare occasions I see the true-life motive for my seemingly unconscious mind striving to work out a problem.

When I'm done torturing my characters and they can have their scene back (or many times the new torture) I'm left with my answer.  How many characters does it take to make a good book?  All of them.  (All that are left, that is.)