Every 210.000 blocks, the block-subsidy gets cut in half, where the subsidy currently sits at 6.25 BTC per block, it'll be 3.125 BTC after the coming halving, and half that after the next halving, up until the block reward is so small that the protocol doesn't register it anymore, which will cause the supply to be close to 21M coins.

It's all written down in the protocols consensus rules.