Different style manuals have different rules that explain when to write out numbers and when to use numerals. If your instructor has designated a particular style guide (MLA, APA, or CMS) for the assignment, then follow the rules for that style. As a general guideline, write out numbers that can be expressed in one or two words, and use numerals for longer or more complex numbers.
Example 1: I have thirty-two cabbages.
Example 2: There are 13,587 cabs in New York.
Technical writing (STEM writing) that involves a lot of numbers should use only numerals both to save space and to make comparisons easier. The biggest thing to remember (regardless of what you are writing) is to maintain consistency. Pick a style and stick to it.