How many Conspiracy Cards are allowed in a deck
The rules for conspiracy cards are listed in section 313 of the comprehensive rules. Two of the relevant rules from that section are


  
  313.1. Conspiracy cards are used only in limited play, particularly in the Conspiracy Draft variant (see rule 905). Conspiracy cards aren’t used in constructed play.
  313.2. At the start of a game, before decks are shuffled, each player may put any number of conspiracy cards from his or her sideboard into the command zone. Conspiracy cards with hidden agenda are put into the command zone face down. (See rule 702.105, “Hidden Agenda.”)
  


So you can have any number, but only in a limited format like draft or sealed. Outside of a format like that, you shouldn't use conspiracies at all.