well max copies in ygo is 3, and in Magic is 4;
I managed to beat this game with with a deck containing ONLY 2 COPIES OF STILETTO, this card is insainely broken and loops into itself.

The consumable food cards helped a lot in creating this clicking frenzy deck.

If you want  to unlock all the cards (if we exclude the value from quests), you have to get 13,025 coins so when you buy them at the beginning of the game be careful cause you will be stuck with that choice for a long time