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davidcyclebackRarity and scarcity are different things. Rarity is only a representation supply (1 card, 3 cards, etc). Scarcity is a measurement of supply versus demand. There is no one way to measure scarcity, as there are different legitimate ways to define demand. However, a standard and common way to measure scarcity in a capitalist society is financial value. The more expensive, the scarcer the card.
For cards that physically exist, the rarest cards will be the ones were only 1 exists. There are numerous examples of these.