But this new coins from the fork will not affect the number of BTC that is on the market right? but why is it affecting the price of BTC whenever a fork is about to happen. Will the new coin also be mine-able or the quantity will be fixed on the quantity that BTC is currently circulating?
It's true that the number of BTC is not affected. But the effect on the price of BTC is essentially the
market pricing in the potential for adverse impacts in the future.
For instance, as many of the previous posts have pointed out, just the fact that BitcoinGold or
BitcoinCash has Bitcoin in the name creates confusion (and therefore potential of negative impacts
on Bitcoin adoption.) There's also a chance (albeit small) that the new coin will actually take over
and become dominant, thereby also reducing the value of Bitcoin.