When to write a number as a numeral and when as a word — it’s a dilemma for writers and editors.
Different style guides have different rules. But no matter what style you use, you eventually get an awkward mix of word and numeral like “He’ll leave in nine or 10 days.” The switch between word and numeral is a bit jarring.
Of course, you could use nothing but numbers. But you’ll get oddities like “She’s the 1 person who can do the job.”
Or you could use nothing but words, but you’ll get clunky verbiage like “The distance to the moon is two hundred thirty thousand one hundred miles.”
I’d write all one-word numbers — three, eleven, nineteen, eighty, and so on — as words.
Numbers that need more than one word — twenty-one, three hundred, fifteen thousand two hundred sixty-two and three-quarters — would be numerals.
Or to put it another way:
All whole numbers from one through twenty would be words;
21 through 29 would be numerals;
thirty would be a word;
31 through 39 would be numerals;
forty would be a word;
and so on.
You still might get an occasional mix of word and numeral like, say, “Our club now numbers twenty or 21.” But that sort of mix would crop up less often than it does with smaller numbers like nine and 10.
P.S. For very big, round numbers, I’d combine numeral and word rather than lay out a numbing, hard-to-parse parade of zeroes and commas. So in writing about how much money Bill Gates has, I’d write it as $90 billion rather than $90,000,000,000.
What say you?