How do I style numbers referring to charts of record sales?
Follow the MLA’s general rule for styling numbers (MLA Handbook, sec. 1.4.1). In discussions where few numbers appear, spell out those that can be written in a word or two (e.g., a number one hit, three top-ten hits). If you refer to sales chart numbers frequently or if you refer to numbers that must be written in more than a word or two, use numerals (e.g., The album entered the chart at number 125, moved to number 90, and eventually reached the number 10 spot).
MLA Handbook. 8th ed., Modern Language Association of America, 2016.