QUOTE(wengherng @ Oct 11 2017, 11:52 PM)
The maximum capped limit of bitcoin is 21 million.
This is a designed limit that is coded into the bitcoin protocol when Satoshi Nakamoto first created bitcoin; it is not a mathematical algorithm.
As such, just like the block size (for example), it is a feature that can theoretically be changed, assuming that each and every miner in the world agrees to the change.
No single entity controls this......it is completely decentralised and any modification requires complete consensus.
If not, and one party still insists to increase the capped limit to 42 million bitcoins, then at some point it will become another hardfork.
But the important thing to also consider here, is that once such a hardfork happens, it just means that another altcoin is created (for example, called Bitcoin42 or something).
It will not be the "original" bitcoin anymore, and as such, the "true" or "original" bitcoin will still have 21 million bitcoins limit, i.e. no change.
Just gonna nitpick and will say it is technically an algorithm, not a constant coded into the protocol.

Basically the algorithm says: For every 210,000 blocks, halve the amount of bitcoins released. If the halving process exceeded 64 (calculated with block height / 210,000), return 0 bitcoins.
Emission rate (over 33 halves, there are 64 as mentioned) is similar to below - Didn't format the figures to reflect satoshis: