Usability testing sample size is defined as the number of users that you compile to test your website or mobile app. When it comes to the ideal sample size for usability testing, we have always recommended that you start with 5 users. Today, we will discuss the factors that affect prototype usability testing sample size. Doing so will allow you to look at the bigger picture before conducting prototype usability testing. You will also get a good idea of the appropriate prototype usability testing sample size for your UX research.