I can get 30
With Black Market you can get as many different Kingdom Cards as you want (limitted of course by the number of different cards in the game, but we wont get anywhere near that number).
In the beginning you have Caravan Guard (played on your opponent's turn), Hireling and Champion in play.
Then you call Ratcactcher, Guide, Transmogrify and Teacher.
Then you play your Princed King's Court and play Throne Room, Procession, Golem, playing 4 different action cards (including a gainer) and calling Coin of the Realm and Duplicate. With the 2nd Procession play you play Herald, playing 2 different cards. With the 2nd Throne Room play you play Graverobber gaining Golem and Herald back from the trash, drawing Herald due to +1 card token on Graverobber. With the 3rd Throne Room play you play Herald, drawing Golem and playing 2 action cards.
Then you call Royal Carriage to replay King's Court and play Golem, playing 6 action cards.
It's still the start of your turn, and you count the cards in play: Caravan Guard, Hireling, Champion, Ratcatcher, Guide, Transmogrify, Teacher, King's Court, Throne Room, Procession, Coin of the Realm, Duplicate, Graverobber, Herald, Royal Carriage, Golem and 14 other action cards gives a total of 30.