This has been questioned (5 user theory). From 2008 research paper: http://www.simplifyinginterfaces.com/wp-content/uploads/2008... Excerpt: Historic reason: Both Nielsen (1993) and Virzi (1992) were writing in a climate in which the concepts of usability were still being introduced..They were striving to lighten the requirements of usability testing in order to make usability practices more attractive to those worki…
Towards the end, we realized the past 20 or so tests have been a waste of time. Issues and improvements that arose from the first 20sh tests kept repeating themselves throughout every remaining session.
This sure increases confidence in your data, but really when you're in an MVP stage or you don't have much funding, you're better off testing with around 15 to 20 people and fix the issues that they find, because most likely, those issues are in fact very problematic and deserve more priority. More users will just yield more granular bugs and issues that you can schedule for later.