Does castling require one card, or two, as you see it? I vote one, a King
card, since officially castling has long been viewed as a move of the
King. But I could go either way.
A 19-card set sounds like a plan...White with nine, Black with nine plus
the Wild card, no card used on White's first move.
I can see some potential for endgame draws, where mating material is
hindered by a lack of sufficient cards to make the moves. :)