Numbers zero through nine should be spelled out and numbers 10 and higher should be written in numeric form, as stated in the Associated Press Stylebook. In the Chicago Manual of Style, whole numbers up to 100 are spelled out, though following the AP rule is also acceptable.
For both writing styles, spell out all numbers that begin a sentence, except for years. Dates, ages, percentages and ratios are usually written in numeric form. Writers should avoid placing two numbers next to one another to prevent a misreading. For example, "12 20-inch tires" would be better written as "twelve 20-inch tires."