We have moved a long way since the simplistic rule of thumb that 5 users is enough. A very good argument in why 10 is not enough is Woolrych and Cockton 2001. They point out an issue in Nielsen's formula (1-(1-0.31)^5) in that he does not take into account the visibility of an issue. They show using only 5 users can significantly under-count even significant usability issues.
The number of users you need is dependent on how many issues there are, the cultural variance of your user base, and the margin of error you are happy with. Testing 5 users (or even 10) is not enough on a modern well-designed web site.
For example, assume that designers of a web site have been using good design principles and therefore an issue only effects 2.5% of users. Then 10 users in a test will only discover that issue 22% of the time. If your site attracts a 1 million visitors a year, 25,000 people will experience problems.
The easy way to think of a Usability Test is as a treasure hunt. If the treasure is very obvious then you will need fewer people, if less obvious then you will need more people. If you increase the area of the hunt, then you will need more people.
For most of the advocates of only testing 5 to 10 users, their experience comes from one country. Behaviour changes significantly country by country, even in Western Europe. See my blog post here :
You may also want to look at the margin of error for the test that you are doing.