There are two rules that people follow:

1. The basic rule is very simple:

- use words for numbers from 0 to 9
- use figures for numbers from 10 on
There were six people at the meeting.
There were 12 people at the meeting.

2. Some people use a slightly different rule:

- use words for numbers from 0 to 99
- use figures for numbers from 100 on
There were sixty people at the meeting.
There were 120 people at the meeting.

Whether you use rule 1 or 2, the important thing is to apply the rule consistently.

There are some exceptions to these rules:

a. If the number comes at the beginning of a sentence, it is normally written as a word:

Two hundred people were present.

b. Dates, times with AM or PM, street numbers, chapter numbers, page numbers, measurements etc. are normally expressed as figures:

January 1, 2010
6:25AM
10 Downing Street
6 kilograms
Chapter 3
page 255

c. Decimals, percentages, degrees and money with fractions are normally expressed as figures:

0.465
7° or 7 degrees
54% or 54 percent
$7.25

Finally, note that all ordinal numbers are normally expressed as words:

twenty-first century
Third Reich
his second wife
Twenty-third Psalm