Okay, to phrase it more clearly: how many characters in a book make too many characters in a book? Obviously if the book is epic in scope, it has to have a lot of people, but good authors help the reader keep track by gently reminding and clearly delineating. A simple statement like "Alex looked at Waverly, wondering what the detective was thinking" can keep a reader from losing track of who's who.
I learned early on that keeping the character list small at the beginning of a story is helpful. The reader needs to learn who the essential characters are before being deluged with secondary and tertiary folks. I also believe that characters added for "color" should be limited to a few well-drawn entries rather than splashed throughout, otherwise the plot becomes difficult to navigate.
This came to mind because the book I'm currently reading strays on all counts from what I just mentioned: too many characters, no reminders of who they are, and lots of supposedly interesting color characters who drop in for a moment and then disappear from the story. If the author were Charles Dickens, I'd have faith that it was all going to come together at some point, but in this case I think the author believes he is delivering a panorama of the era. Instead I find myself wondering, "Now is Annabelle the sister, the aunt, or one of the servants?"