How many characters should be in a novel?

I got asked the above question today in email, and here's my reply:

The smallest number of characters with which you can effectively tell the story. If you have multiple minor characters who can be consolidated into one, do so.

Classic example: the original Star Trek pilot. The character of Number One lacked emotions; the character of Spock was alien. When they were combined, the show became better.

Or Next Generation, first season: the security chief (Tasha Yar) was a separate character from the Klingon (Worf), and the latter didn't really have anything to do. When they were combined, the show became better.

Of course, another answer is at least 240,000 -- that being the average number of typed characters in 40,000 words, which is the shortest length that qualifies as a novel in the science-fiction industry ... ;)

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