I like to have designated carry and practice mags. Practicing reloads and malfunction clearing can be tough on mags, even if you consistently use some kind of padding to drop them on.
I like 6, preferably 8, mags. When I had a Glock 23, I had 6 mags...3 for carry, 3 for practice. All 6 were function-tested in the gun.
I currently have a Beretta 92FS, but it fills a HD role so I don't carry it, at least not yet. I have 4 function-tested 15-round mags, so 2 are set aside for practice with one loaded mag in the gun and another loaded mag as a spare. I recently got some 18-round MecGar mags, but I haven't had a chance to function-test them yet.
As for carry, always at least one reload, revolver or semi-auto. However, I tend towards carrying 2 reloads, regardless of capacity, for 2 reasons. 1) While having to perform a reload in the middle of a gunfight is highly unlikely, if I do need to perform one it may be possible to fumble the reload. I like the idea of being able to just grab a second reload off my belt rather than pick the dropped mag off the ground. And B) I found that carrying 2 reloads on my belt allows my cover garment to drape a little better, making them less obvious. Another, lesser, reason for carrying 2 reloads is that it may better balance the weight of your carry gun if you carry reloads on your left hip and your gun on your right hip.
And I always carry at least one reload because if I am forced to fire in self defense, I don't want to wait for the police to respond with an empty or partially empty gun. 911 response times in my area averages around 9 minutes. With semi-autos, it's also important in the event of a malfunction as magazines can be the weak link when it comes to reliability.
As for the total number of mags to own, I'd recommend as many as you can. Not just for any possible future legal restrictions, but also because mags are essentially consumables, like tires on a car. Use them until they stop working properly, then replace them.