There's 21 millions of Bitcoin and not any less or more. That's as simple as this, so no, it can't fluctuate.
This is the second time that I have seen the same mistake on this thread, theres is cap of 21 million coins but those 21 million coins have not been mined yet, only when all coins have been mined is when the number of coins will not fluctuate.
Get the total number of bitcoin does not mean anything. According to forecasts, bitcoin in the bank still will not be 21 million, because a large percentage of the loss will be. But even this moment will bring rapid growth of prices for bitcoin in the future.