When it comes to writing numbers, the general rule of thumb is to use numerals to express numbers 10 and above, while words are used to express numbers below 10. For example:
You may have to consider on a case-by-case basis if an exception to the rule applies. For more details, consult the APA Style page on expressing numbers. Sections 6.32 and 6.33 of the Publication Manual of the American Psychological Association cover how to express numbers.