The source you are documenting may be part of a sequence, like a numbered volume, issue, episode, or season. A text too long to be printed in one book, for instance, is issued in multiple volumes, which may be numbered.
If you consult one volume of a numbered multi-volume set, indicate the volume number in the entry. (p. 158)
Here are two examples:
Rampersad, Arnold. The Life of Langston Hughes. 2nd ed., vol. 2, Oxford UP, 2002.
Wellek, Rene. A History of Modern Criticism, 1750-1950. Vol. 5, Yale UP, 1986.
Journal issues are typically numbered. Some journals use both volume and issues numbers. In general, the issues of a journal published in a single year compose one volume.
Usually, volumes are numbered sequentially, while the numbering of issues starts over with 1 in each new volume. (pp. 158-159)
Here is an example:
Baron, Naomi S. "Redefining Reading: The Impact of Digital Communication Media." PMLA, vol. 128, no. 1, Jan. 2013, pp. 193-200.
Other journals do not use volume numbers but instead number all the issues in sequence.
Here is an example:
Kafka, Ben. "The Demon of Writing: Paperwork, Public Safety, and the Reign of Terror." Representations, no. 98, 2007, pp. 1-24.