Arcana
All 15 arcana
Fifteen cards
You start with The Cursed and the Cursed Flame spell. Boss cards are The Fallen (Holy Cross and Arcane Whip), The Warrior (Infernal Pyre and Holy Light), The Prophetess (Petrifying Gaze), The Judge from Saint Louis, The Occultist (Blood Spikes, with Aerial Rush and the paintbrush on the same clear), The Builder (Fist of Talos and Smash), The Artist (Masterpiece and Aerial Rush), The Miser (Wheel of Chaos), The Brittle King (Stained-Glass Shield), The Temptress (Life Drain), The Self (Absolution), The Shadow (Soul Harvest and double jump), and The Zealot (Chronomantic Dive).
The fifteenth is the ferryman's card, The Whisperer, after the Miser line: the three eyes, Mammon, and the coins you hand the ferryman. Map pins show 14 arcana because the starting card is not a pin. Both numbers are right if you say which list you mean.
Cards the ending checks
The good ending wants the Four Virtues and a finished The Self. Works of Mercy on a card are separate tasks: the ending notes mention killing four enemies with one spell five times, exploring 85 percent of the map, and filling your health with that spell ten times. Those tasks are not the same as simply owning the card.
What I ran into
I counted cards on the pause screen, got 15, then opened a map that said 14 and thought I had missed a room. The missing pin was the card I started with. That footnote is the reason the page exists.
Questions
How many arcana are there?
15, counting The Cursed. Map markers show 14 because the starting card is not placed in a room.
Which cards are required for the true ending?
The Four Virtues and The Self, with The Self's Works of Mercy finished. Owning them is not the same as finishing the tasks.