An oft-revisited topic, one of perpetual interest. If you go here
and scroll down to "Lifetime of opened bottles / Whisky & storage", you'll find a lot of discussion on this. Long story short, yes, whisky will change over time after opening, sometimes to its benefit, somes to its detriment. How many to have open at a time is matter of personal choice, to some extent--some here have only a few open at a time, others have dozens. Bottles that are getting low bear watching, and it's widely agreed that when you get down to the last few drams, it's best not to leave the bottle sitting for any length of time. Many have reported saving that last dram of Glen Googly Special Edition for a special occasion, only to be disappointed by a flat and lifeless drink. Resist the urge to save it, or maybe pour the last 10cl into sample bottles to reduce contact with air. We've had lengthy arguments about whether deterioration is caused by oxidation or evaporation, and I don't want to start that again, but we all agree that deterioration does occur in such circumstances.