Number is for count nouns, and amount is for mass nouns, but some usage guides include exceptions to this rule. What are the exceptions? Should you take note or ignore them?
A sentence in a Slate story jumped out at me this morning because of the way the writer used amount and number. These two words are not interchangeable—they mean different things.
- Number is for things you can count such as pebbles, people, users, and timelines.
- Amount is for things you can't count such as anxiety, faith, water, and heartache.
Another way to say this is that number is for count nouns and amount is for mass nouns.
The morning mix-up stood out strongly to me because the writer used amount and number interchangeably in a sentence with parallel structure. He used both amount and number to refer to things that can be counted: users and timelines:
What Do the Style Books and Usage Guides Say About Amount and Number?
Garner's Modern American Usage, Chicago Manual of Style, and Hacker's The Writer's Reference state the countable/not countable rule succinctly and without exceptions.
Merriam-Webster's Dictionary of English Usage (MWDEU) and Fowler's Modern English Usage address the rule but add that in common usage, writers sometimes use amount with plural count nouns "when they are thought of as an aggregate"—which could cover the Slate writer's use of amount to refer to users in the example. Both MWDEU and Fowler's show examples in which writers use amount to refer to people in a crowd. MWDEU only notes the usage; Fowler's bemoans it. Garner's examples of incorrect usage also include amount to refer to people: The amount of ex-players who talked shows that the authors did their homework.
Still, in the Slate article, I wonder what makes users an aggregate and timelines individually countable? The parallel nature of the sentence makes the use of these two words particularly jarring to me.
Despite MWDEU and Fowler's inclusion of the exception for plural count nouns thought of as an aggregate, I recommend sticking with number when referring to things you can count. It is the recommended style in many style guides, and many readers believe it is a rule. Therefore, using amount to refer to count nouns, such as users, is likely to be distracting.
In the bigger picture, the Slate article has great insights about the difference between Twitter and Facebook. You should read it: Twitter is not dying. It's on the cusp of getting bigger.