“Fewer” is correct, but it’s wrong to correct “less” to “fewer”. Why? Because language isn’t dictated by an authority, it is negotiated theough billions of interactions every day.

People say “less” to refer to countable objects and nobody is confused by it, so it is valid communication. Just because you had some English teacher mark you down for it in middle school doesn’t mean it’s wrong. You were just miseducated.

Even if we’re talking about strict technical semantics, 5 is less than 7. The < symbol is not called the “fewer than” symbol. “Less” can encompass the concept of “fewer”, so either is fine.