Numbers can be a tough nut to crack for a writer.
Not only do the accepted rules vary between style guides (like so many other things), but some of these guidelines have various exceptions which further complicate the matter.
So, let’s start with the basics.
It’s generally accepted that the numbers one through nine are written out regardless of context. After that, the writer is expected to shift to numerals unless the number is the first word in a sentence or is following such an example (or use of a number that falls between one and nine). However (and again, this is where things get tricky), one can argue this practice has deeper roots as a standard for technical writing than fiction.
Research also indicates that it’s often acceptable to write out any number that consists of only one word (ten, twenty, thirty, etc.), and perhaps even anything lower than a hundred (although, that may be a bit of a stretch). Additionally, numbers should be written out when they appear in any form of dialogue (conversation or quotes) or are being used to round (“about 100 million people”).
To sum things up: Any number lower than ten or put between quotation marks should definitely be written out. A number starting a sentence should be written out, and any other number sharing that sentence with it should also be expressed with words. And generally, it’s agreed that any number that can’t be expressed in one word (due to hyphenation and/or size) should be written as numerals.
The key here is consistency. Whatever rule you follow, be sure to apply it to the entire manuscript. Just don’t express numbers different ways within the same sentence.