According to the MLA Handbook for Writers of Research Papers (Seventh edition), for most MLA style papers, spell out numbers written in one or two words such as "two" or "three hundred." If you begin a sentence with a number, spell it out, even if it is a date or other number that uses more than two words.
When using large numbers, as long as they do not start the sentence, you can use a combination of numerals and words, such as "3.6 billion." Write out spans of time such as "sixteenth century." Use numerals for large numbers that do not allow for a smooth combination of numbers and words, such as "151." In papers that rely heavily on numbers, focusing on mathematical or scientific concepts or containing many statistics, use numerals before measurement terms, such as "3 centimeters" and data comparisons such as "the scores rose from 7 to 12." Use numbers to represent amounts of money, in addresses, with symbols such as "%" and "lb.," in decimal representations and for specific dates.
[Note: There are many exceptions to the rules about how to write numbers.
These tips will point you in the right direction, but if you are serious about understanding all the rules, you need to buy a style guide such as The Chicago Manual of Style or .] Whether to use a numeral or to spell out a number as a word is a matter of style.
Typically, people who write business or technical documents are more likely to use numerals liberally, whereas people who write less technical documents are more likely to write out the words for numbers.
If someone handles numbers a different way than you do, they're probably using a different style guide, so the best advice I can give you is to pick a style and stick with it when it makes sense.
Use the same structure for all portions of ideas that demonstrate a comparative relationship, such as "4 of the 310 members." Use commas between every three digits from the right in large numerals ("1,000" and "352,000,000").
Type symbols such as "$" for money amounts and percentages in MLA.
(Since I used to be a technical writer, I write out the words for numbers one through nine, and use numerals for most other numbers.) Fortunately, some rules about writing numbers are more universally agreed upon than the general rules I just told you about.
Let's say you’re writing about snail development--a technical subject--and you've decided on a style that says you use words for the numbers one through nine and numerals for anything bigger.