The requirement for 52 card decks is spelled out in section 523 where it discusses equipment conditions. The pertinent paragraph is copied below:
523.15. Cards; physical characteristics.
(a) Cards used to play table games authorized under this subpart, must be in decks of 52 cards with each card identical in size and shape to every other card in the deck. Nothing in this section shall prohibit a manufacturer from manufacturing decks of cards with one or more jokers contained therein; provided, however, that the jokers may not be used by the certificate holder in the play of any game unless authorized by the rules of the game.
(b) Each deck must be composed of cards in four suits: diamonds, spades, clubs and hearts.
(c) Each suit shall be composed of 13 cards: ace, king, queen, jack, 10, 9, 8, 7, 6, 5, 4, 3, 2. The face of the ace, king, queen, jack and 10 may contain an additional marking, as approved by the Bureau of Gaming Operations, which will permit a dealer, prior to exposing his hole card at the game of Blackjack, to determine if the value of the hole card gives the dealer a Blackjack.