Bitcoin has a fixed monetary base of 21 million BTC, which will never increase or decrease. These coins are initially distributed (as mining rewards) at a limited and constantly decreasing rate, and once they have all been distributed, no new coins will ever exist. However, this will not be the end of Bitcoin, because the initial distribution of new bitcoins is not the only reward for mining: miners also collect transaction fees. Although transaction fees currently only make up a small fraction of the total mining reward, this will change as Bitcoin becomes more popular, since more transactions means more total fees (assuming the fee per transaction stays the same).

The number of coins in circulation (as opposed to the total number in existence) will slowly decrease over time as people accidentally lose them, however it is virtually impossible for all the bitcoins to disappear (since that would require that everyone loses everything), and because Bitcoins are divisible the economy can continue to function normally even if only a single bitcoin, or a fraction of one, remains.