If it is an e commerce site you need a very powerful database. The webservers won't be the bottleneck.
The best way is to profile. That means measure how much cpu time/memory and database resource an average page view takes. But since web sites don't go from 0 to 200 million page views per day I seriously doubt you will need all the infrastructure up front. AND it will be prohibitivly expensive; thousands upon thousands per month.
So don't worry about it right now. Just get a moderately powerful web server and separate database server. Unless you are advertising on the super bowl. In that case, go hire a few programmers, some IT guys and a ecommerce consultant or tow cause you will need them.