To uncover user experience issues in a product, we recommend five participants per study. It’s more effective to do a series of small tests at multiple points throughout development, rather than one large study at the end. That helps you identify problems early, when it’s easier to fix them. You can start testing an app or website as soon as you have concept sketches or wireframes.
If you’re looking for trends and insights beyond basic usability issues, we recommend five to eight participants per audience segment.