Assuming it's like most tabletop RPGs, the answer is probably "one." Three to six is more typical, though.
While it's a guess, appears to be less than five, since the "Pregenerated Character Sheets" from the downloads page includes only five characters:
http://cryptorpg.com/pregen_sheets.pdf
How do you play a tabletop RPG by yourself?
Agree, though appears to be a thing; here's a pretty good guide to "solo rpg/roleplaying":