Grammar rules about when to spell out whole numbers (one, two, three) and when to use numerals (1, 2, 3) vary greatly from publishing house to publishing house. It’s really a matter of style.
Most do agree that we should spell out numbers zero through nine. Most also agree that all negative numbers should take numerals (-1, -2, -3).
The disagreement primarily is over numbers larger than nine. The Associated Press Stylebook calls for using numerals (10, 11, 12, 100) while The Chicago Manual of Style says to spelling out numbers up to and including 100 (ten, eleven, twelve, one hundred) also is fine, giving you the option to choose between two styles.
A number of exceptions exist to these basic rules, most obviously when a number starts a sentence (then always spell out the number) or when giving addresses (then always use numerals).
Your best option is to consult with your publishing house for a list of their style rules using numbers. If you’re self-publishing, adopt either AP or Chicago style and then be consistent in following it.
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My name is Rob Bignell. I’m an affordable, professional editor who runs Inventing Reality Editing Service, which meets the manuscript needs of writers both new and published. I also offer a variety of self-publishing services. During the past decade, I’ve helped more than 300 novelists and nonfiction authors obtain their publishing dreams at reasonable prices. I’m also the author of the 7 Minutes a Day… writing guidebooks, four nonfiction hiking guidebook series, and the literary novel Windmill. Several of my short stories in the literary and science fiction genres also have been published.