I used to be really compulsive about always knowing how much ammo I had. But with multiple guns in multiple calibers and frequent range trips and ammo purchases, I would need an automated inventory tracking system to keep a definitive count. The numbers aren't particularly high, they just change a lot.
But as a rough SWAG, I'd say I have about 2000 rnds for each rifle, 1000 rnds for each pistol, 500 for each shotgun and boatloads of rimfire.
Under normal circumstances, you probably don't need to have too much set aside. And I don't anticipate any sweeping changes in ammo purchasing laws (despite the threat of a bullet tax in the PRK). But it's nice to be able to (a) save money by buying in cases, (b) save time by not having to pick up ammo for each trip to the range, (c) be prepared just in case the S does HTF, (d) have a stock as a hedge against delivery problems to your local gun shop.
By the media's standards, I guess I'm a gun wacko. By my friends' standards, I'm a piker.