The Efficient Cloud: All Of Salesforce Runs On Only 1,000 Servers
http://www.techcrunch.com/2009/03/23/the-efficient-cloud-all-of-salesforce-runs-on-only-1000-servers/

Salesforce - incredible efficiency! and other metrics

all of Salesforce.com runs on only about 1,000 servers. And that is mirrored, so it is really only 500