It does not affect the maximum suppply of Bitcoin. If you fork Bitcoin there would be an altcoins (Name of the fork) + Bitcoin) They are trying to make a 2.0 Version of bitcoin. It would not really change anything with Bitcoin it self.
You really don't get it do you?
A person wants to invest 100 USD of real money in cryptocurrencies. If there was only one crypto - Bitcoin - with a limited supply, then over time you can argue price will go up as long as that person really want to by crypto for his USD. But when you then have more and more cryptocurrencies, some of the people will choose to put their 100USD in competing cryptos. And this means in reality that the supply of crypto currency is growing exponentially and without limits