Don't need to read it again.
"how many headphones" is correct.
It's just the "much" at the end of the question that's wrong.
Because "how much headphone is too much" doesn't make sense - there is only one correct answer.
If the question was about pie, instead of headphones, then it works because it's inferred in the question that either a number of pies or a large pie is consumed.
But the question is about headphone(s) and putting a singular headphone in a question of quantity is incorrect. That's like saying "how much lamp is too much?" which is just wrong.