How do cards with "X" work?

How do cards with "X" work? - Close-up of Coin

I couldn't find any explanation inside the game. Maybe it's the number of the turns passed since the beginning of the current combat?

slay-the-spire

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How do cards with "X" work? - Sale Cards on Beige Background

How do cards with "X" work? - Four Ace Game Cards

How do cards with "X" work? - White Printer Paper on White and Brown Floral Textile

Cards x Coins | NO SETUP Self Working Card Trick REVEALED!

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Images: Michael Steinberg, Karolina Grabowska, Pixabay, Karolina Grabowska