According to APA Style, you should normally use numerals for the numbers 10 and above. You should use words to express numbers below 10 (one, two, three, four, five, six, seven, eight, and nine).
Note, however, that APA lists a wide variety of exceptions. For instance, numbers that appear right before a unit of measurement should be expressed as numerals, regardless of where they fall, before or after 10. Example: Give him 5 cups of soup.
The APA also addresses this question directly in this webpage.
The sections in APA that address numbers are 6.32-6.39 in the 7th edition of the Publication Manual.
