The limit of 21 Million is set in the code which can only be changed with a hard fork, which would require agreement of the community.
It may worth mentioning that there is no "21 millions" in bitcoin code.
The total supply of approximately 21 million comes from the initial block reward of 50 BTC and halving each 210,000 block. The exact supply of bitcoin is 20,999,999.9769 BTC.