As a gun collector, I have to keep track of ammunition if ever I want to shoot my guns. As of last count, I have handguns in twenty-four different calibers, and long guns in another twenty-eight.
Most of those calibers, however, are ones for which I have no real need to stock up. For instance, I doubt I'm going to get the sudden urge to run out and shoot a thousand rounds through my Carcano carbine tomorrow, so the couple boxes of 6.5 Carcano I have are plenty. I have dies for it, too, so I'll make sure to save the brass.
Some military surplus calibers I've stocked up on more considerably; after all, when you come across good deals on surplus .30-'06, 8mm Mauser, or 7.62x54R, it's silly not to sock them away, especially if you have multiple guns in the caliber.
The commercial ammunition is almost all stored in the factory boxes or MTM Casegard containers, except for five ammo cans: These are full of loose .22LR, .38 Special, 9mm, .45 ACP, and 5.56 NATO for easy transport to the range. I need to add one more for .44 Special, and I'll have all my working calibers covered. (Note that that handily covers small, medium, and large bore calibers for both revolvers and semiautos, and my house gun.)
90% of my shooting is done with just those calibers, and pretty much 100% of my serious shooting is. I don't know exactly that it's an urge to simplify, like Xavier, but I will say that over time I've gotten a lot less enthralled with the search for the perfect caliber; these days a majority of the preening that goes on over caliber esoterica strikes me as folks who want to show how cool they are by how outside the mainstream their caliber choice is. Sure, 9x23 has impressive ballistics; of course it's a shame .41AE didn't catch on; yeah, I've got a 10mm handgun, too, but I can shoot my .45 a whole lot more...