From the Intrigue Rulebook:
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To set up for 5 or 6 players, combine the Treasure cards from Dominion and Dominion: Intrigue. Use 15 Provinces in the Supply for a 5-player game and 18 Provinces in the Supply for a 6-player game. All other Victory card piles (Estates, Duchies, and Victory Kingdom cards) remain at 12 cards per pile.
I was spectating a 6P game and noticed there were 16 Provinces, Duchies, and Estates, and the regular 30/40/60-starting_decks treasures instead of what the rulebook states.