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.