When you use only one volume of a multivolume work, include the volume number of the volume you used and page numbers of article cited.

Hopkinson, Tom."George Orwell." British Writers. Ed. Ian Scott-
     Kilvert.Vol.7. New York: Scribner's, 1984.273-87. Print.

When you use more than one volume of a multivolume work, cite the number of volumes in total (10 vols.) after title.

McDonald, William. Opposing Viewpoints in Montana History. 3 vols.
     Greenhaven: San Diego, 2002. Print.