"they probably don't have 20,000."
http://linux.omnipotent.net/article.php?article_id=12019
"Linux clusters are here to stay. Like Linux itself they are playing an important role in our daily life without our even being aware of them. For example google.com (my favorite search engine) runs a Linux cluster of around seventy thousand machines! That's one of the reasons why Google is so fast. "
And this was from 2001!
The author Satyakam Goswami didn't provide a source.