The auto-numbering wizard available via Convert → Auto-numbering wizard... Ctrl+k allows for assigning tracknumbers and discnumbers to the selected files.

Tracknumbers

Begin at tracknumber

Sets the start index for numbering.

Save total count of tracks

If enabled, also saves the total tracks number. This field is prefilled with the selected files count.

Leading zeros for tracknumbers

If enabled, pads the tracknumber with leading zeros.

Please note that, e.g., MP4 tags store the tracknumber as integer value instead of a numeric string, so that leading zeros are not stored. Use the $num scripting function when renaming to pad with leading zeros.

Discnumbers

Begin at discnumber

Sets the start index for numbering.

Save total count of discs

If enabled, also saves the total discs number.

Leading zeros for discnumbers

If enabled, pads the discnumber with leading zeros. The same notes on tag format support apply as for tracknumbers.

Resetting and Incrementing Counters

Next disc on change of directory

If enabled, the track counter is reset and the disc counter is incremented on directory changes.

Next disc on change of field

If enabled, the track counter is reset and the disc counter is incremented if the value of the selected field changes.

Reset disc counter on change of field

If enabled, the disc counter is reset if the value of the selected field changes.