As Vincent said, using the disc numbers is a good and fully functional solution for albums that consist of more than one disc.
How do you add double, tripple and boxsets, and how do you add the covers when each CD have different covers?
When you have a box set that consists of more than one album with individual cover art you will need to tag the albums with individual album names if you want to show them separately in views that are grouped by album.
For example, the album tags for "The Beatles Stereo Box Set" (which contains 14 remastered original albums) could be:
The Beatles Stereo Box Set - Please Please Me
The Beatles Stereo Box Set - With The Beatles
The Beatles Stereo Box Set - A Hard Day's Night
etc
or perhaps
Please Please Me (Stereo Box Set version)
With The Beatles (Stereo Box Set version)
A Hard Day's Night (Stereo Box Set version)
etc
or just (if you don't have any other versions)
Please Please Me
With The Beatles
A Hard Day's Night
etc
+ "From The Beatles Stereo Box Set" in the Comment field.
EDIT
I forgot to mention that "The Beatles Stereo Box Set" contains also two 2-disc albums (The Beatles and Past Masters). It is easy to group these two albums correctly with disc numbers.
There is no need to add the disc number "1" to the 12 other albums in the box set (or to any single-disc album in general). The built-in logic works fine when only the multi-disc albums have disc numbers.