Spolsky recommended at least one dedicated tester for every two or three programmers. Without them you either ship buggy products or waste money by having $100-an-hour programmers do work that $30-an-hour testers can do (his 2000 figures).
The ratio has shifted because much repetitive checking is automated. Developers write unit tests with Vitest 102,989 or Jest 62,362 and end-to-end tests with Playwright 26,280 or Cypress 41,181 (Testing Frameworks), and CI runs them on every push. Human testers are still essential for what automation misses: exploratory testing, confusing flows, accessibility with real screen readers, and odd devices. A passing suite only proves the code does what the tests expect; a curious human notices when the tests expect the wrong thing.