If you find your self needing to write a number, you might question whether you should spell it out, used the word or you should use the numeral. Well, this is really a matter of style so it depends on who your writing for and what's style guide your following but one of the most common style guide is the associated press style book and so I'll give you the AP rule, but just note that whenever your this you might wanna look up the style of which ever publication you are writing for.
The AP style book, suggest that whenever you have a number one trough nine you spell it out and when you have a number great than nine you used the numeral. When will you can remember this is that if you have a number that is at least two digits like 10, we wanna use the numeral but if we have a number that's one digit like let's say seven, we wanna write it out.
So the AP does two exceptions, they say that whenever your writing an age or measurement you should use the numeral, so that's even if the age is two digits or more, say your wanna say that someone is 13 years old, you would write "He is 13 years old" Using the numeral, if I wanna say "He is 6 feet tall" It would look like this, again were using the numeral and in this case were hyphenating 6 and feet because there forming an adjective phrase to describe how tall he is. I hope that, that gives you some good information about how to use numbers, when to spell them out and when to use the numeral.
Always remember to check the style guide that your using and if you don't use a style guide, the important thing is that you be consistent trough out what ever your writing.