Jakob Nielsen’s Alertbox, June 4, 2012
If you want a single number, the answer is simple: test 5 users in a usability study. This lets you find almost as many usability problems as you’d find using many more test participants.
This answer has been the same since I started promoting “discount usability engineering” in 1989. Doesn’t matter whether you test websites, intranets, PC applications, or mobile apps. With 5 users, you almost always get close to user testing’s maximum benefit-cost ratio.
As with any human factors issue, however, there are exceptions: