2 player game; no possession; no durations; 5 card hands; normal turn.
In my action phase, I play a single action card. By the time it's resolved, 15 cards have been gained. How?
Side puzzle: 15 was the best I could do. Is it possible to do better?
All 14 Estates have been Ambassadored back into the supply (thanks to Lighthouse).
Play Black Market, playing Platinum, Venture (revealing Philosopher's Stone), and Horn of Plenty. Horn of Plenty gains a Cultist, which is trashed to my revealed Watchtower, drawing 3 cards. I play a Copper, Counterfeit a Horn of Plenty, gaining 2 Border Villages, and gaining and trashing 2 Cultists. I do that 3 more times, and play a Copper. I have 5 Horns of Plenty in my hand, a Watchtower, 6 Counterfeits, and 4 other cards. I Counterfeit 2 Horns of Plenty, gaining and trashing 4 Hunting Grounds for 12 Estates. I Counterfeit 3 Horns of Plenty to gain 6 Catacombs (2 with Border Villages first), trashing for 6 Squires, trashing for 6 Familiars. I then buy a Scout from the Black Market deck.
I gained 9 Cultists, 10 Border Villages, 12 Estates, 4 Hunting Grounds, 6 Catacombs, 6 Squires, 6 Familiars, and a Scout, for a total of 53 cards gained from playing a single Action card.
EDIT: Crap, I have to fit in Market Squares and Tunnels in there too. Also, Catacombs should gain Feodums to trash for Silver.