Spell out numbers one through nine. Any number over that should be typed numerically (10, 11, 12, etc.). Exceptions to this rule:
Dates
Time
Addresses / Phone Numbers
Dollar amounts
If a sentence begins with a number, spell it out! Unless it’s a date, time, address, phone number, or dollar amount.
If a sentence contains a number that should be spelled out (one through nine) and a larger number (10 or more) that should be written numerically, both should be written numerically for consistency.
In the case of a sentence that starts with a number and later comes another number, spell out both numbers for consistency.
Examples:
I have four kids.
There were 12 victims hospitalized.
Twelve of the victims were sent to the hospital.
After the first 5 pages of the document, you could see where the story was going, even though it was over 250 pages long.