The total amount of Bitcoin is capped at just under 21 million in the rules. New Bitcoin come into being only as a reward for miners, and this reward is cut in half about every four years. If you add up all the rewards, the sum approaches the limit without ever exceeding it. The limit is enforced by the nodes: every block that creates more new Bitcoin than allowed is rejected. The last Bitcoin are expected to be created around the year 2140.
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