MC11 has a [disc #] field.
After populating this, it can be used in any filenaming mask.
to prevent breaking MC's [complete album] logic you have two choices:
If you keep all the tracks in a single directory, then you must number all the tracks sequentially.
If you would rather keep the track numbers on a per-disc basis, and maintain the complete album logic, then you need to place each disc in its own folder.
Using mesue's example above, the mask would become:
home folder\[artist]\[album]\[disc #]\[track #] [name].mp3
When using the rename from properties tool, the following expression will cover both single and multi-disc albums, simply paste it into the directory rule field. If your home folder\ is not My Music, remember to edit the string accordingly.
if(isempty([disc #],1),My Music\[artist]\[Album],My Music\[artist]\[Album]\Disc [disc #])
-marko.