Servers are so damn powerful these days that it's a bit misleading to use the number of servers as a measurement. The standard a few years ago was dual processor PIII ~1Ghz machines with 1-4GB memory. These days it's a 4+ core monster with 8GB+ memory.
Most of the sites these days with a ton of servers just have Google envy or investors who want money to be "put to work". Server and network gear is a really easy way to go through money.
We launched our site with only one machine (dedicated host I think we paid about $100/mo for it as well) and had stories about us on the big blogs (TechCrunch, Gigaom, etc) - it performed like a champ. I guess it depends on what your app is like though.
Awesome, good to know.. I was a little nervous that I didn't have a massive cluster ready to go ;) I'm just going to make sure my database indices are in the correct places and see how launch does.
Steve Huffman of Reddit posted on their latest server stats, which is interesting: http://reddit.com/info/2gdcv/comments/c2geky
Most of the sites these days with a ton of servers just have Google envy or investors who want money to be "put to work". Server and network gear is a really easy way to go through money.