Count Nouns and Mass Nouns
The fastest way to advertise your ignorance of the English language (or your fatigue) is to misuse a homonym. The second fastest is to use a mass-noun adjective on a count noun. And unlike the homonyms, you can make this error in speech as well as writing.
Ninety-nine percent of the problem comes down to two adjectives: Less and Amount. Those adjectives only apply to mass nouns, which are nouns that don’t come in discrete units. Count nouns, which do come in discrete units, use two often-neglected adjectives: Fewer and Number.
So install a little warning light in your head that flashes when you’re about to speak or write those two danger words: Less and Amount. Stop and ask if the noun you’re about to describe presents itself as separate items or an undifferentiated mass. If it does, then stop and find the right word.
Whether something is a mass or count noun isn’t a physical property, but instead a feature of how a particular noun conceives of its subject. Often you can describe the same physical stuff as separate units or an undifferentiated mass, depending on the noun you pick:
A number of dollars; an amount of money.
Less time; fewer minutes.