But Twitter needs more than 5 employees.

How many bug reports do you think the Twitter site and apps get each day? How many employees do you think it takes just to triage them?

You don't have to read all of the bug reports, just a representative sample to discover issues worth fixing.

How do you get a 'representative' sample?

You still have to read them to work out that user X saying they can't log in is the same as user Y saying authentication is failing at A stage, issue is present on B and C platforms version D.E

Further the issue of 'site using the wrong font' is not equally as important as 'site is exposing private user data'. And you're likely to get many people reporting the former, and maybe only one reporting the latter, so a sample isn't likely to catch the really important stuff.