The total supply is an infinite series of halves.

1/2 + 1/4 + 1/8 + 1/16 ... On to infinity. Add all of them up and they'll never equal more than 1 whole.

The whole supply of 21,000,000 can never be exceeded as the supply is generated as an infinite series of halves every 4 years.

First four years 10,500,000 coins were mined (1/2 of 21,000,000) and the first halving cut the issuance rate in half, which also cut the total issued over the following 4 year period by half as well. So the second set of 4 years see 1/4 of the total supply (5,250,000 coins) issued which now totals 1/2 + 1/4 = 3/4 of total issued in the first 2 periods. This continues until the code doesn't allow the halving to continue (currently 2140, so we have time).

Essentially, the total number of the supply will always be approaching 21,000,000 but it will never actually reach 21,000,000.