Number is the sixth core element. This element may not be necessary for every Works Cited entry. Number is used to document multiple volume works which are numbered. For example, journals are typically numbered by volume and issue.
Book in multiple volumes:
Damrosch, David, et al, editors. The Longman Anthology of World Literature. 2nd ed., vol. A, Pearson Education, 2009.
Houtman, Eveline. "Mind-Blowing: Fostering Self-Regulated Learning in Information Literacy Instruction." Communications in Information Literacy, vol.9, no. 1, 2015, pp. 6-18. www.comminfolit.org/index.phpjournal=cil&page=article&op=view&path%5B%5D=v9i1p6&path%5B%5D=203.
Comic book issue:
Byrne, John and Jim Lee. "The Resurrection and the Flesh." X-Men, vol. 2, no. 4, Marvel Comics, Jan. 1992.
Television series episode: