Google has posted a short but interesting video to its Webmaster Central YouTube channel. A user asked the question, "How many bots/spiders does Google currently have crawling the web?" and Google's Matt Cutts gave his answer.
"It's important to realize that it's not really actual robots or actual spiders out there...instead, it's banks of machines ...at Google's data centers who open up an HTTP connection and request a page and then get it back," he says. "So any bank of machines (even 50 machines) could easily be requesting a bunch of different content."
"We try to refresh a large fraction of the web every few days," he adds. "So it turns out you really don't need a ton of machines. Even a relatively small amount of machines operating in parallel and fetching pages in parallel can really be able to crawl of find new pages on the web in a very quick way."
Matt says that Google doesn't give out the exact number, but that it's somewhere between 25 and 1,000. I'm not sure what you can really do with that information, but it's worth hearing a quick rundown of how it works for those who aren't real familiar with how Google indexes content.