The general guideline is to use the percentage symbol with numerals and to use the word percent with spelled-out numbers.

In statistical copy that calls for frequent use of numbers, it’s appropriate to use numerals, and so the percentage symbol would be used, as in the following example, drawn from a report on a census of language enrollments:

Japanese enrollments increased by 3.1%, from 66,771 in 2013 to 68,810 in 2016; Korean enrollments increased by 13.7%, from 12,256 in 2013 to 13,936 in 2016. (Looney and Lusin 3)

In prose that does not make extensive use of numbers, as in the example below, drawn from a keynote speech, numbers are spelled out, and so the word percent is used instead of the percentage symbol:  

The results were somewhat surprising but should have been predictable given the change in student demographics. Only four percent of those responding planned to continue to graduate school in Spanish or related disciplines. (Compitello 91)

Section 1.4.1 of the MLA Handbook offers further guidance on when to use numerals and when to spell out numbers in your work.

Works Cited

Compitello, Malcolm A. “From the Classroom to the World and Back Again: Cultural Studies as Mediator of Curricular and Global Change.” ADFL Bulletin, vol. 44, no. 2, 2018, pp. 90–101, doi:10.1632/adfl.44.2.90.

Looney, Dennis, and Natalia Lusin. Enrollments in Languages Other Than English in United States Institutions of Higher Education, Summer 2016 and Fall 2016: Preliminary Report. Modern Language Association, Feb. 2018, www.mla.org/content/download/83540/2197676/2016-Enrollments-Short-Report.pdf.