This is what the MLA Handbook for Writers of Research Papers had to say on the subject of numbers:
For Literature and Other Infrequent Number Subjects
"If you are writing about literature or another subject that involves infrequent use of numbers, you may spell out numbers written in one or two words" (such as one, thirty-six, ninety-nine, one hundred, fifteen hundred, etc.) and "represent other numbers by numerals" (such as 2 1/2, 101, 137, 1275, etc.).
To form the plural of a spelled out number, treat it like an ordinary noun. (such as sixes, sevens, etc.)
For Science and Statistics
"If your project is one that calls for frequent use of numbers, say a paper on a scientific subject of a study of statistical findings, uses numerals for all numbers that precede technical units of measurement" (such as 16 amperes, 5 milliliters, etc.).
For Inclusive Numbers
In a range of numbers, use numbers only and give the second number in full for numbers through ninety-nine.
For example: 2-3, 21-47, 10-12, 89-99
For larger numbers, give only the last two digits of the second number, unless more are necessary.
For example: 96-101, 103-04, 395-401, 923-1001, 1003-05, 1608-774
For additional information on numbers, including Roman numerals percentages, and money, see pages 81-85 in the MLA Handbook for Writers of Research Papers.
