Jared Spool argues that the 5 users myth doesn’t apply to all user research methods. The statement that including 5 to 8 users in your study is enough to discover about 85% of usability issues is only applicable to usability testing. However, mature companies shift towards more proactive research in order to get a better understanding of their users. Instead of focussing on reactive research (=usability testing).
For proactive research the 5-8 user myth doesn’t apply.