Getting more than one dedicated server for starting a site (no matter if you expect it to grow to size of SO over time) would be a waste. (Consider that SO itself has taken years to grow to its present size).
I would even suggest start with a VPS (virtual private server). If there are even the slightest hint of growth, you can go for dedicated.
You might have heard of cloud computing. It works out better (cost and ease of scaling) in the long run for some type of high traffic sites but is more expensive (both in terms of cost and bootstrapping time) to start off with it.
Might be interesting and even helpful to you: Check out the Alexa traffic history graph of stackoverflow: http://www.alexa.com/siteinfo/stackoverflow.com# (mind you these are not accurate for small sites but fairly okay for large sites). Go to Reach or TrafficRank or PageViews and select Max in the drop down menu to see the nice, rapidly increasing curve from 2009 to 2010. If you are really curious, you can check out graphs for some recently successful web startups.