According to Turabian's A Manual for Writers (a compendium of CMOS), the rules for determining whether words or numerals should be used are as follows:
- Spell out numbers from one through one hundred; if the number has two words (such as thirty-five), use a hyphen.
- Spell out round numbers like thousand, hundred thousand, million, etc.
- Never begin a sentence with a numeral; always spell an initial number out.
- Use numerals for percents and fractions (except at the beginning of a sentence; use the word percent instead)
- In a series of large and small numbers, use numerals (e.g., 153 children and 5 adults...)
- Spell out currency units (e.g., twelve dollars and three cents)
These are just general rules, and only for CMOS. If you require a specific style guide, consult that instead. Additionally, these rules do not apply for finance/math/statistics papers, or other settings involving many numbers or tables.