So there are 21 million total because that is how it was coded. Will it be possible to increase the supply or even make it infinite if someone was able to crack the code?
Isn't about a user cracking the code, to apply a change on the network then more than 50% of the nodes should run with that change. In theory, this could be possible, but more than 50% of the people should be agreed with this idea and that's the complex part.
When governments print more money the coins lose their value, that could happen to bitcoin if a fork like that comes.
So, my answer is, yes, it can be manipulated but it will never happen.
Precisely, if competent enough the closest you can get is to create a fork and hope other people will see value in that newly created fork and use those coins to derive value from but since this is a commonly known idea among the crypto geeks and community in general nobody is going to fall for that. At least not enough people for the fork to become mainstream.
So, you need not to worry about BTC having an increased supply than its cap already!