1. Spell out numbers of one or two words or those that begin a sentence. Use figures for numbers that require more than two words to spell out.
ie: Now, some eight years later, Muffin is still with us.
I counted 176 CD’s on the shelf.
2. If a sentence begins with a number, spell out the number or rewrite the sentence.
ie: One hundred and fifty children in our program need expensive dental treatment.
Or rewrite of this sentence:
In our program 150 children need expensive dental treatment.
In technical and some business writing, figures are preferred even when spellings would be brief but usage varies.
When several numbers appear in the same passage, many writers choose consistency rather than a strict adherence to the rule.
When one number immediately follows another, spell out one and use figures for the other: three 100-metre events, 60 four-poster beds.
GENERALLY , figures are acceptable for dates, addresses, percentages, fractions, decimals, scores, statistics and other numerical results, exact amounts of money, divisions of books and plays, pages, identification numbers and the time.
In some cases (mostly with money and in contracts) both numbers and words are used - words followed by numbers in brackets would apply.
From: A Canadian Writer’s Reference - Second Edition by Diana Hacker