Total supply below 21m

Even the theoretical maximum supply is below 21 million: because subsidies are integers of sats and halvings truncate, the schedule sums to approximately 20,999,999.9769 BTC. The realized total is lower still. The genesis block's 50 BTC coinbase is not in the UTXO set (Bitcoin Core treats it as a special case, so it can never be spent), and some early miners claimed less than the full permitted subsidy in their coinbase transactions, permanently forgoing the difference.

Nuance: These reductions (a few dozen BTC of unclaimed rewards, 50 BTC genesis) are distinct from lost coins, where the coins exist in the UTXO set but the keys are gone (see lost-coins-estimates). Whether the genesis quirk was intentional is not established.

Common misstatements:

• "Exactly 21 million bitcoin will exist." — The schedule itself tops out at ~20,999,999.977, and unclaimed rewards push the spendable total lower.
• "Satoshi burned the genesis reward as a statement." — The genesis output is unspendable due to how the client handles it; intent is undocumented.