A lot according to who you talk to. In fact, the question reminds of the fablesd, "how many licks does it take to get to the center of a Tootsie Roll Tootsie Pop?". The wise answered, "Three".
Looking of the world of sample testing, there are several views how many participants are enough to get a valid result. Let's start with What’s in a Number? By Carol M. Barnum. She states, "Whereas 7 (plus or minus 2) is the mantra for structured writing and other methods for organizing information, 5 (plus or minus 2) is the mantra for the number of participants needed in a usability test." They added that 5 will yield 80% of findings, although now they have lost faith in this number when it comes to testing Web sites. They said it will take more than 5 to uncover all the usability issues.
Jakob Nielsen thinks it should be more segregated into four different areas: heuristic, usability, participants and scenarios to gather a more comprehensive view of the web site. Nielsen says that a substantial portion of the user population should be used to correlate your findings.
The article also mentions combining methods when testing. I like this idea the best. I am a firm believer in mixing-and-matching to get the most of everything. This is what I will do.
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