Because táo could mean the apple trees or anything apple related, so we need quả to be more specific (the apple fruits).
It would be helpful if there was a rule which explains bao nhieu.
My answer was correct, 'how many'..... However, I could also see 'how much are the apples you have'. As an example, if I replaced apples with golf clubs, they'd both be acceptable questions yet how to differentiate the bao nhieu meaning.
Just a thought. Thanks for feedback.