Fixed ceiling
The protocol limits cumulative issuance. Unlike fiat currencies, there is no discretionary monetary authority that can simply increase the maximum supply.
Monetary policy
Why 21 million
Bitcoin?
Bitcoin’s supply policy is encoded in its consensus rules: block rewards decline over time and total issuance converges on 21 million BTC.
Protocol maximum
21,000,000
BTC — fixed maximum supply under Bitcoin’s current consensus rules.
Halving schedule
The subsidy is reduced approximately every four years, lowering the rate at which new bitcoin is created.
Finite, not immediate
Bitcoin did not begin with all 21 million coins. Supply is released over a long issuance schedule that extends far into the future.
Consensus matters
The 21 million limit is enforced by network rules. Changing it would require broad adoption of incompatible consensus changes.
Scarcity is a protocol property.
The significance of 21 million is less about the exact integer and more about credible scarcity: users can independently verify the issuance rules and total supply constraints.