Here you seem to contradict yourself in consecutive sentences. In the first sentence you say "zero bananas" is "not a number of bananas". But In the second sentence you tell us that zero is a number after all. If zero is a number, like 1 or 3 or 17, then "zero bananas" must be just like "one banana" or "seventeen bananas". It's a number of bananas. Webster's - like most dictionaries - first and foremost records common usage, not mathematically rigorous definitions.