Should I spell out a number or not?

The suggestion I’ve seen for fiction is to spell out numbers that are one hundred or less. For something like 153,760,888 write it as numerals.

Since years tend to fit the “greater than one hundred” rule, write 2020 rather twenty-twenty.

But for “round large numbers” like 50,000 I tend to write “fifty thousand”. Yet in the year 2000, I still wrote “2000” not “two thousand”.

There probably is a fuller set of rules to be googled up.

I'm no expert in rules, but I think the main thing to consider is how easy it is to read. If it's awkward written in words then write it as figures. If the figures look awkward then write it using a combination of figures and words.

For example:

Text: ten, three, twelve, a million, ten billion

Digits: 34,600; 14,000; 692.4; 0.0035

Combination: 324 trillion (because 324,000,000,000 and three hundred and twenty four trillion both look yuck)

I think you can't really go wrong by picking the most readable version. If someone comes up with more detailed rules, then go with that. I'm no expert. But any rules really ought to be based on readability or what's the point of them?

Consider writing the cards as e.g. three of spades, two of diamonds, ten of clubs. Also, from my experience of being 5'1" and constantly asked for ID everywhere*, bouncers and bar staff will just straight up ask for ID. That might eliminate needing to write the necessary age out in words or digits.

*well, when I was young. These days I just look short and middle aged lol.

On the freelance transcription gig I do on weekends, official style says to spell out any number up to ten, then use numerals. But that's nonfiction, technically.

The key in any work is consistency... and, of course, readability.

The reason you found conflicting info is because there's different types of "standard" writing styles/formats. You might remember MLA from high school, and there's stuff like APA, Chicago, AP and even things like the New York Times and NPR have their own internal style guides, too. Some require oxford comma, for instance, and others don't.

For my current WIP I have one-twenty, 21+ except one hundred, one thousand, etc. Ages would be e.g. 2 years old, years are also numbers (2020). Why? I don't know that's what I picked. Whatever you pick just write it down for your own internal style guide and just be consistent. There aren't phone numbers or money or anything, but those I'd have as numbers since that's what people normally call them (911 not nine-one-one, your total came out to $3.50, not three-fifty).

So for your examples I'd do
21
$25
two glasses
cards 10, 3, 2

Don't stress about it too much. There's no right answer here.

AP (associated Press), CMS (Chicago Manual of Style, (NYT) New York Times... Every publication has a different style. I began with AP and used it for two decades before anyone told me anything dfferent, so I gravitate to that. Numbers 0-10 are spelled out, over that are numeric, unless a number list includes both ranges, in which it is all numeric. CMS is 1-100 spelled out.

The beauty is, after your manuscript is accepted, the publisher's editor will correct whatever you screw up.

Jeff

Just pick a rule set and follow it. If you're trade published, your editor will know the house style and respond accordingly. If you're self-published you'll have made your work easier by following a standard. Either way, if you are consistent, it's easier to change later if you need to.