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Source: https://writers.stackexchange.com/a/16714
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

In fiction, there's no rule for this, only differing styles and opinions. However, some editors seem to like using the Chicago Manual of Style's alternative rule for this.

> **9.3**  **_An alternative rule--zero through nine._** Many publications, including those in scientific or journalistic contexts, follow the simple rule of spelling out only single-digit numbers and using numerals for all others [...]

It goes on to reference the exceptions to this. (9.3 is an alternative to the more strict rule 9.2 that requires spelling out all numerals from zero through one hundred.)

Exceptions to these include spelling out round numbers. For example: "seven hundred", "one thousand", "forty-seven thousand", etc. There are also other rules for money, ordinal numbers, decimals, percentages, etc, but these are overkill for fiction.

The number you indicate in your question is more like a serial number. I can't find anything for that, but personally I'd use the abbreviated rule for numerals in general, using numerals and letters and not spelling them out. Unless the character's name were something short like the robots in Star Wars, where (for example) R2-D2 becomes "Artoo" for short.

Original score: 5