Complex sentence: The volume number was incremented at the start of a calendar year, regardless of the number of issues, with volume one, 1950, having only two issues; subsequent volumes varied from five to twelve issues depending on the magazine's frequency of publication.
Split into simpler sentences: The volume number rose by one at the start of each calendar year, regardless of the number of issues. Volume 1,1950, contained only two issues; subsequent volumes contained five to twelve issues, depending on frequency of publication.