You Only Need to Test with 5 Users (2000)
Some key questions:1. How extensively do those 5 people test the software? Do they test all features or just part of the software?2. What is the background of those 5 people testing the software? Do they understand UX/good UI design and how well?3. Are these 5 people just random users or professional test engineers?4. How passionate are these 5 people about the product/service they are testing? How meaningful is it for them that the product actually works _really well_?5. What is the quality level of feedback these 5 people can provide? Is it like "meh, this is ugly" or is it detailed, concrete and contains practical improvement ideas that can be easily implemented?