18.
If I were to run a Halloween team, here are my planned choices.
1. Ariados (Spider)
2. Crobat (Bat)
Noivern and Gliscor also make for perfect bat candidates, although the Zubat line are irreplaceable and a classic that's simple to obtain.
3. Silvally (Frankenstein's Monster)
4. Mismagius (Witch)
5. Gourgeist (Pumpkin)
6. Mimikyu (Bedsheet ghost)
Honourable mentions:
-Mr. Mime (Evil clown)
-Gengar (General spoopy ghost)
-Dusclops (Mummy)
-Parasect (Zombie)
-The Impidimp line (Devil)
-Lycanroc Midnight (Werewolf)
-Dusknoir (Bogeyman)
-Alolan Marowak and/or Eternatus (Skeletons)
-Trevenant (Haunted dead tree)