As a journalism major in college, I was already familiar with AP style when I started writing resumes. Using AP style, numbers are expressed as follows:
• Numbers from 1 through 9 are spelled out:
Supervised four employees.
• Use figures for numbers 10 and above:
Hired 14-20 seasonal staff each summer.
But I had a little more trouble being consistent with the following rule:
• Use the same style to express related numbers above and below 10. If any of the numbers are above 10, use figures for all numbers.
Example: Recruited, hired, and trained 6 line managers, 11 assistant managers, and 87 part-time and full-time employees.