Tatoeba contains 2 sententes with the same translations:
Is there difference between these sentences? What measure words can be used with 苹果?
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Common measure words for apple are: 个，颗，粒
A more appropriate sentence would be,
"几 + measure word + 苹果?"
While for the other case,
Is totally fine asking just like that, as it is plainly asking how much apples are there. When someone ask the question "多少苹果?", the reply could be using various measure words such as, 箱 (a box), 篮 (a basket) or 一袋 (a bag) etc.
While "几 + measure word + 苹果?" would limits the answer to the certain measure word.
A: "请问这里有几箱苹果?" B: "我们店里还有十二箱苹果！你要多少苹果？" A:"那你就给我十二颗苹果吧!" B:"什么？就十二颗？我还以为你要几十箱呢！"