Question: Can a book have too many characters?Answer:
It really doesn't matter how many characters a book has as long as they are engaging.
You need at least one main character whose dilemma the reader can relate to, and an impact character who presents an alternative way to handle the problem (even if this character is imaginary). That's the minimum.
Some novels have many, many characters, if they are long and have a number of subplots. (The Harry Potter series is a recent example.) It all depends how many you need to tell the story or to flesh out the main character's world.
Of course, there are some practical considerations, for instance...
* You must keep your plot moving.
* Publishers may be more inclined to publish a 100,000 word novel by a first-time writer than a 600,000 word novel (which means less space for extra characters).
* It will be easier for you to keep track of 20 characters than 200, and easier for the reader too.
* You don't want redundant characters. If two characters fulfill basically the same function, it may make sense to combine them.
But there's no hard and fast rule. It all depends on your story.