The answer, according to Mel Blanc himself, was about 400. I know because I worked for Mel as his Studio Operations Manager in the early 1970's. One day, while setting up his mic in the voice booth, he volunteered the comment that, while he was known as "The Man of 1,000 Voices," the true number of distinctly individual characters he had voiced by that time was around 400.
Mel Blanc Audiomedia was in an office building at the corner of Wilshire Blvd. and Rodeo Drive in Beverly Hills, CA. I worked directly for Mel as his recording engineer and audio producer under the direction of his very talented son, Noel Blanc. MBA was renowned for creating brilliant radio commercials and syndicated radio programming, such as the "The Gary Owens Special Report" and "Superfun," a package of short comedy bits featuring Mel that also included Joanie Gerber's hilarious "Story Lady."
I continue in the business to this day (2007) as a voiceover performer and VO teacher based in Philadelphia. I hope you found thse comments to be informative. For more on working with Mel Blanc, I invite you to read the stories on my website, Voiceoverisland.com. Thank you. Chuck McKibben - phillyvo.net; (215) 677-2295.