The supply is determined by the coin creator... Network isn't actually completely decentralized as most coins have at least two seed nodes that makes it possible for all other nodes to find each other. Seed nodes contain identical copies of blockchain and all other nodes must agree to that version of blockchain or there will be a fork consisting of one or more alternative chains.
Interesting, do you have any reference to this bit of information?
https://github.com/forknote/forknote/blob/master/README.md