Contextual Inquiry is a interview method to uncover who users really are and how they work on a day-to-day basis. This creates a empirical basis for the later design.
Amount of people
User (or usability) testing is a process of evaluating a solution idea for a future product or service to see how well it fits the problem or need of the indented target users. The aim of user testing is further more to gather feedback and insights from users in order to improve the user experience, usability and functionality.