Generally, you should spell out numbers one through nine and use numerals for 10 and above.
* Example: "Three cats were playing in the garden."
* Example: "There were 12 people at the party."
Exceptions:
* If the number is part of a measurement, date, time, or percentage, use numerals.
* Example: "The book is 5 inches thick."
* Example: "The meeting is at 2:00 PM."
* If the number is the first word in a sentence, spell it out.
* Example: "Seventeen students signed up for the trip."
* Example: "Two hundred people attended the concert."
* If the number is a year, use numerals.
* Example: "The event took place in 2023."
* Follow the style guide specific to your writing. Some style guides have specific rules for numbers, such as those for academic papers, journalism, or technical writing.
In short, there's no one-size-fits-all answer. Use your judgment and consider the context to decide whether to spell out a number or use numerals.