コピー is almost a verb; it's closer to "copying" than "how many copies." The counter for CD's and DVD's is 枚(mai), 冊(satsu) for books, 本(hon) for tapes, and, hmm, dunno for game carts (I'd guess 個(ko)).
I'd explain the word コピー"kopi".
Japanese "kopi", coming from English "copy", means only "machine-copying" or "machine-copied documents". We don't use the word for the copied things other than paper/documents such as CDs.
For example, if you say 本のコピー(hon-no kopi), which can be directly translated to "a copy of book" , actually means "a document made by copying some pages of the book".
Here is some counters of Japanese:
枚(mai) for thin things such as paper, CDs, cloth, etc.
冊(satsu) for books and book-shaped things such as notebooks, diaries, etc.
本(hon) for slender, cylinder-like things such as pens, sticks, bottles, etc.
台(dai) for vehicles and machines.
個(ko) is used for many things, sometimes instead of other counters.