Style and context matter when you're using numbers in a sentence.
Numbers Next to Each Other
Here's another one most people seem to agree on: When you are writing two numbers right next to each other, you should use words for one of them and a numeral for the other because that makes it a lot easier to read. For example, if you write,
"We tested 52 twelve-inch snails,"
you should write the number 52, but spell out twelve (or vice versa).
Beginning of a Sentence
When you put a number at the beginning of a sentence, most sources recommend writing out the words. If the number would be ridiculously long if you wrote out the words, you should rephrase the sentence so the number doesn't come at the beginning. For example, this sentence would be hard to read if you wrote out the number:
Twelve thousand eight hundred forty-two people attended the parade.
It's better to rephrase the sentence to read something like this:
The parade was attended by 12,842 people.
The second sentence uses passive voice, which I generally discourage, but passive voice is better than writing out a humongous number and taking the risk that your readers' brains will be numb by the time they get to the verb.
Some style guides make an exception to allow you to use the numeral when you're putting a year at the beginning of a sentence, but others recommend that you use words even in the case of years.
Next: Numbers in Dialogue