In academic writing, spell out numbers of one or two words (twelve countries, twenty-two thousand people). A hyphenated number may be considered one word.
In most business writing, use numerals for all numbers over ten (12 countries).
Round dollar or cent amounts of only a few words may be expressed in words (fifteen dollars).
When the word o'clock is used for the time of day, express the number in words (nine o'clock).
Spell out numbers that begin sentences. If the number requires more than two words, reword the sentence so that the number falls later in the sentence and can be expressed as a numeral.