Bitcoin’s often-quoted 21 million limit emerges from the initial 50 BTC subsidy, halvings every 210,000 blocks and integer accounting in satoshis. Total possible issuance is slightly below exactly 21 million BTC.
The limit is credible only because users run software that checks issuance. A code change proposing more coins would not alter existing nodes; acceptance would require users to adopt incompatible rules.
The issuance cap is not the same as available supply. Keys can be lost or coins made provably unspendable, reducing effective circulation. The network cannot distinguish intentional long-term holding from inaccessible keys.
Derived from subsidy math
Slightly less than exactly 21 million
Enforced by validating nodes
Lost coins reduce effective supply