counter
counter for sets of flat items
Used to count collections or assemblies of flat objects, such as CD sets, DVD box sets, or photo collections. Often appears as ~枚組.
このCDは2枚組です。
This CD is a 2-disc set.
3枚組のDVDボックスを買った。
I bought a 3-disc DVD box set.
枚 is the basic counter for single flat objects; 枚組 counts sets or collections of such objects.
Compound of 枚 (counter for flat objects) and 組 (set, group).