The FBI is assuming 53, but that implies 2 Jokers in the deck, which is interesting but unfounded.
The Jack called his driver "Two" but it's occurring to me that Jacks can outrank Aces in euchre decks, which only have 24 cards. The "Two" doesn't fit that, but the Jack sure acted like he was in charge at Zia's.
Tarot decks have 78, and stuff that trumps aces, but the suits are different. Anyhow, 53 of 54 is an assumption, not something we're sure about.
EDIT: Oh I see our author said the 53, but that was in the comment section, so does that count as canon?
p.p.s : @empath, there's hostages that need to be rescued, and readers that need catharsis, so hope the writing is going well.