This isn't even including any possible characters. Which even then sounds daunting to add. Most of this is purely conjecture.
Curse Kastle Boss: Only mentioned as Him by the bat.
The Miner: Someone has to be collecting those crystals in the background and possibly humming "Hi Ho". It may also enlighten us on the nature of the ghost.
Doorstore Employee(?): While we can always have faceless characters, it would be interesting to have an employee of Doorstore. Just an idea of the character.
The Collection Facility Worker: Basically the guy who you talk to for the semen extraction.
The Hospital Receptionist: Consider how the Rhino is... there must be someone to set him straight.
Those are all the characters I can think of which bumps our total number of minimum characters to about... Twenty... 24.
I am torn between being excited to the stories of the characters to the agony of having to draw for those characters. On one hand that means we can have loads of content on the other hand hopefully you don't experience burnout.