When I was in school, I was taught to write out the words for small numbers, and to use numerals for any number which takes more than three words to say, which means any number over one hundred gets numerals, not words. For example: "The facility generated 2,400 gallons of wastewater during the past twenty-one days."
At times, though, people will write out a number in words, then add numerals in parentheses for clarity. The above example would be written as: "The facility generated two thousand four hundred (2,400) gallons of wastewater during the past twenty-one (21) days."
I can see the value in this format, particularly when the numbers are very large and complicated to write out, but I have seen this taken to silly extremes. I once saw a letter which read: "We should be hearing back from the town in a day or two (2)."
So, does anyone here know of a rule of thumb for this sort of thing?