If there's multiple discs, prepend tracks with the disc number

.config/beets/config.yaml

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+directory: /stuff/beets/music
+library: /stuff/beets/musiclibrary.db
+
+art_filename: cover
+asciify_paths: yes
+per_disc_numbering: yes
+threaded: yes
+
+paths:
+  # default: $albumartist/$year - $album/$track $artist - $title
+  default: %the{$albumartist}/$year - $album%aunique{}/%if{$multidisc,$disc-}$track $artist - $title
+  singleton: Singletons/$artist - $title
+  comp: Compilations/$album%aunique{}/%if{$multidisc,$disc-}$track $artist - $title
+  albumtype:soundtrack: Soundtracks/$album%aunique{}/%if{$multidisc,$disc-}$track $artist - $title
+
+import:
+  write: yes
+  copy: yes
+  move: no
+  resume: ask
+  #incremental: yes
+  log: beetslog.log
+
+ignore: .AppleDouble ._* *~ .DS_Store
+
+plugins: [
+  discogs,
+  duplicates,
+  embedart,
+  fetchart,
+  ftintitle,
+  importadded,
+  inline,
+  the,
+]
+
+discogs:
+  # pip install discogs-client
+  # Useful for classical music:
+  index_tracks: yes
+
+fetchart:
+  # pip install requests
+  cautious: true
+
+ftintitle:
+  auto: yes
+  format: (feat. {0})
+
+importadded:
+  preserve_mtimes: yes
+  preserve_write_mtimes: yes
+
+item_fields:
+  multidisc: 1 if disctotal > 1 else 0