Here's an article that can help with that question. It's called 10 Rules for Writing Numbers and Numerals and is found on DailyWritingTips.com.
Here are some of the rules:
- Numbers versus numerals. The number is the quantity, the numeral is the symbol.
- Spell out smaller numbers. Some sources say anything under 10, some day anything under 100.
- There aren't any other rules. All the "official" sources disagree. However, I definitely lean toward writing out.
- Use the comma and the decimal. but know how to use them right. The US and Europe use them in opposite ways; know which one you need.
- Never start a sentence with a numeral. First words are always spelled out.
- Spell out centuries and decades. It's the eighties, not the 80's.
- Spell out related words. Use 5 percent, instead of 5%.
- Spell out estimations. About four thousand, not about 4000.
- Two numbers next to each other. I had a party with eight 12-year-olds
- Spell out ordinal numbers. That was the tenth rule.
I hope y'all find this helpful; I did.