Jakob Nielsen’s study on the ideal number of test subjects in usability tests found that tests with just five users would reveal about 85% of all problems with your website, whereas 15 users would find pretty much all problems.
Source: Jakob Nielsen’s AlertBox
The biggest issues are usually discovered by the first one or two users, and the following testers confirm these issues and discover the remaining minor issues. Only two test users would likely find half the problems on your website. This means that testing doesn’t have to be extensive or expensive to yield good results. The biggest gains are achieved when going from 0 test users to 1, so don’t be afraid of doing too little: any testing is better than none.