You can check the source code yourself, it's open source.
static const CAmount MAX_MONEY = 21000000 * COIN;
From:
https://github.com/bitcoin/bitcoin/blob/master/src/amount.hAs for changing the number, you'd have to convince everyone that it'd be a good idea to change the number, which I highly highly doubt. I really don't think it's something that you should be worried about.
that's not the place to look, check the comment right above that line.
the fact is that maximum supply as a hard coded number actually doesn't exist anywhere in the code but instead it is mathematically enforced by using the starting supply (in block #0) and halving rules (reward halves every 210000 blocks).
Probably not hardcoded to allow for change via consensus?
I wonder if Op is trying to know whether the supply can't be alter without consensus, or if we can actually verify that extra Bitcoin hasn't been printed in some ways that is unknown to developers/community.
That is an interesting question! Will probably be hard to do that on Bitcoin than other cryptocurrencies