If there are two numbers next to each other, use a combination of numerals and characters for clarification.
   There were twenty 6-foot-high posts used for the fence.
   They gave away 123 ten-page booklets.

If a sentence starts with a number and another number is separated by no more than three words, use text for both. Otherwise a number starting a sentence has no affect on the other number styles.
   Thirty men and forty women attended the event.
   Fifty years ago they saw an eclipse and the next one won’t occur for another 40 years.

Spell out fractions standing alone or followed by of a, of an, or of the.
   The gap was three fourths of an inch.

Hyphenate fractions when they are used as adjectives and spelled out.
   There was a two-thirds majority in favor.

Use numerals for mixed fractions unless they begin a sentence.
   The recipe calls for  3 1/2 tablespoons of oil.
   Four and one-quarter percent is the current interest rate.

Use numerals for numbers through the thousands.
   There were 3,450 people in attendance.

Use text for large round numbers.
   The city has more than one hundred thousand inhabitants.

Use numerals and text in numbers in the millions or larger.
   The population of the United States is almost 304.1 million.

Use commas for numbers with more than three digits.
   The book has 1,234 pages.

Spell out one through nine. Use numerals for 10 and above.
   There are three rules listed in this blog post.
   We will probably cover over 12 rules concerning number usage.

If a sentence or paragraph contains multiple numbers applying to the same category, treat them consistently. If one must use numerals, then all should use numerals.
   In three months, I presented 85 grammar posts and 5 non-grammar posts.

Spell out a number at the beginning of the sentence or rephrase the sentence.

   Two-hundred people attended the concert.
   The concert was attended by 250 people.