The actual number of Bitcoins is slightly less than 21 million due to the way the Bitcoin code works. Here's a concise explanation:

The Bitcoin code controls the inflation schedule by defining how many new Bitcoins are created every 210,000 blocks, with the subsidy halving approximately every four years. After 33 halvings, the subsidy for miners becomes 0. Adding up all these subsidies over time results in approximately 20,999,999.9769 Bitcoins, not exactly 21 million.[1][4]

The Bitcoin code contains a constant called "MAX_MONEY," which is set at 21 million Bitcoins, but this is more of a safety check than a strict limit. While the 21 million figure is a good approximation, it's essential to understand that Bitcoin's supply isn't fixed at that number in the code itself.[4]

Sources [1] What Happens to Bitcoin After All 21 Million Are Mined? - Investopedia https://www.investopedia.com/tech/what-happens-bitcoin-after-21-million-mined/ [2] 21 million is relatively small : r/Bitcoin - Reddit https://www.reddit.com/r/Bitcoin/comments/16mgget/21_million_is_relatively_small/ [3] Bitcoin's 21 million limit is a boomer myth - Blockworks https://blockworks.co/news/bitcoins-21-million-limit-myth [4] The 21 Million Bitcoin Myth: Understanding the Code Behind It https://blog.bitwage.com/21-million-bitcoin-myth/ [5] What actually caps Bitcoin's supply at 21 million? https://bitcoin.stackexchange.com/questions/100639/what-actually-caps-bitcoins-supply-at-21-million