According to Nielsen Norman Group, five users is the sweet spot for testing the usability of your product. You can learn 80% of what you need to know about how people will respond to your product from just five users, allowing you to quickly receive feedback and iterate before putting your improved design in front of five new users.
As Nielsen details in their article, you can get far more value from three iterative tests of five users than one test with fifteen users.
The article, written in 2000, was directed at web design projects, but the rule of 5 users is still common practice today — and not just for digital products. User testing can be even more important for physical products where high tooling costs can magnify the expense of a mistake. That is why we often push for our clients to get something in front of users as quickly as possible. Investing in a realistic prototype that can be tested by potential customers can save tens of thousands in capital costs down the road, because changing a design is far more expensive once the required tooling has been built.