Why "How Many Users" is Just the Wrong Question
http://www.humanfactors.com/downloads/may07.asp#kath

Straub, Kathleen
UI Design Newsletter
2007

Abstract:

Every day in offices around the world usability professionals ask and are asked this question: How many users do we need for our usability test? Its an important question. We want to find most of and the most severe problems. So, we need to test enough people. But usability testing is so expensive, and the cost of testing increases with each participant. So, we don't want to test too many, either.