Yes, forking is not mean to increase the total supply but just maintenance for bitcoin like make transaction quicker for example.
Forks (in this case: hardforks) don't change anything regarding bitcoin. It generates a new Altcoin with the same block history until forking block of BTC.
This Altcoin will have the same features as bitcoin plus the freshly implemented (like 8mb blocks in BCC).
May i know what it mean bitcoin supply is set in stone? Does it mean it not possible to break it?
Yes, its not possible to "break" the maximum supply because its hardcoded into bitcoin itself.
The only "option" would be to create a hard fork.. but this, as already mentioned, would create a new altcoin and would have nothing in common with bitcoin.