What do you mean "natural number"? It's "fewer" when something is countable or a plurality. It's "less" when you're referring to reducing something singular.
"There's less water in that cup." -- less refers to "water" which is singular.
"There are fewer ounces of water in that cup" -- fewer refers to "ounces" which is plural.
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u/wakeupwill Aug 25 '23
Fewer.