Are they compute nodes? Are they webservers(5)? Webservers and appservers(10)? Web, app, database(12)? Web, app, database, back-end processing(15)? How much redundancy is built in(30)? Staging environments(60)? Development environments(120)? Standard IT infrastructure like DNS and email(125)?
"We run over 500 servers" sounds a lot like "we have 500 servers in our datacenters" not "it takes 500 servers to handle 200MM page views."