Find the mistake:
Every year, an enormous number of paper is used to produce books.
My thoughts so far:
After googling I found that when using "a number of" we use plural verb (i.e., are). Is that a general rule? In this sentence, paper is uncountable, so is that why we can't use "are"?
The book's answer indicates that we can't use "a number of" with (uncountable), so we must replace it with (an amount of / a quantity of), although in this Macmillan dictionary entry number is described as
[Countable / Uncountable ] a quantity of people or things
Is that right? If so, why?