Though the 21 million supply was never mentioned in the code of the bitcoin, calculating the block rewards certainly yields the number nearing 21 million.
But as far as we know, satoshi in his announcement of the version 0.0.1 has said that the circulation would be always 21 millions and when this runs out the nodes will mine the blocks for the transaction fees.
Total circulation will be 21,000,000 coins. It'll be distributed
to network nodes when they make blocks, with the amount cut in half
every 4 years.
first 4 years: 10,500,000 coins
next 4 years: 5,250,000 coins
next 4 years: 2,625,000 coins
next 4 years: 1,312,500 coins
etc...
When that runs out, the system can support transaction fees if
needed. It's based on open market competition, and there will
probably always be nodes willing to process transactions for free.
Also, as seoincorporation pointed out, the price wouldn't be stable for the miners to mine the coins at a loss along with the difficulty. The difficulty adjusts every now and then with miners not able to profitably mine the coin leaving the market and new ones entering. Never think of the total supply as just 21 million. If each satoshi is worth more or less of a dollar, then total bitcoins value is insanely very high. I have read somewhere that, satoshi designed the system keeping in mind the inflation rates. The fiat money is infinite and hence if more numbers are required they can be printed and the price wouldn't change. Bitcoin is inversely proportional to it. The more demand rises, the price of the coin rises simultaneously since no other extra coins would be created.