Satoshi is able to modify this number.
Developers can modify it and that is called Hard fork.
Satoshi cannot modify this number and make everyone accept it. Anyone with decent technical knowledge can modify the number but they cannot expect everyone to follow it.
The maximum number of bitcoins that will ever be in circulation is 20999999.9769, just shy of 21 million. This number cannot be changed in vanilla bitcoin, but may vary in derivative coins or forked coins.
It's actually a lot less than that. Factoring out the lost Bitcoins, the maximum possible number of Bitcoins to be generated is roughly 20999827.243BTC.