No, it doesn't.
That's like saying "Will LiteCoin reduce the amount of Bitcoin?"
If there is a fork, it doesn't care about the other fork, it's really just as easy as that.
Well, this doesn't make sense to me in the way I think about things.
If HYPOTHETICALLY at "Zero Hour" the hardfork occurs, and XT wins, but I subsequently choose to run my 1 million Bitcoins onto the old Core Chain - and if everyone seems pretty clear that those Bitcoins can NEVER EVER EVER go back onto the XT Chain - then it seems to me that the XT Chain just lost 1 million bitcoins.
So is my understanding of this flawed? Or are you saying that my Core-Forked Bitcoin are still counted in the 21 Million XT Bitcoins - but just "lost".
They exist on both chains.
The only thing, that might happen, is that they never get accessed on one fork, if the people who have the private key, don't want to use their private keys on that chain.
So, you could call that lost, like if somebody lost his private key by crashing his hard drive or to keep up my LiteCoin-analogy: It's like when somebody thinks LiteCoin are much cooler and don't access his Bitcoin anymore.