Beyond the Five-User Assumption: Benefits of Increased Sample Sizes in Usability Testing
http://www.geocities.com/faulknerusability/Faulkner_BRMIC_Vol35.pdf
  peer-reviewed

Faulkner, Laura
Behavior Research Methods, Instruments and Computers
2003

Abstract:

It is widely assumed that 5 participants suffice for usability testing. In this study, 60 users were tested and random sets of 5 or more were sampled from the whole, to demonstrate the risks of using only 5 participants and the benefits of using more. Some of the randomly selected sets of 5 participants found 99% of the problems; other sets found only 55%. With 10 users, the lowest percentage of problems revealed by any one set was increased to 80%, and with 20 users, to 95%.