There are two different types of usability test and each has different aims. […] I’m going to call the first kind of test ‘formative testing’ since its purpose is to shape or mould the user interface; and the second kind of test ‘summative’ testing since its aim is to summarise. […] The ‘magic number 5’ question has been discussed to death with formative tests. […] But 5 is clearly insufficient for a summative test. […] Jakob Nielsen recommends 20 for these types of test; when testing everyday products for usability, ISO 20282 recommends a sample size of 50.
