In v.3.27a I was not able to make Autonumbering Wizard work with discnumber. Whatever I tried, it set numbers for all disks to 1, regardless to the directory changes. I tried with both FLAC and ID3v2.3 tags, no difference.
My settings in the wizard are as follows:
The results are as follows, note that the directories are different for 2 disks here, but the discnumber is set to 1.

