I have just under 2,000 rounds of shotgun ammo on hand right now. Various gauges (.410, 20, 16 and 12). As I mainly use it for hunting and some target / sporting clay shooting I mainly have shot shells and not much in slugs or buckshot. I have some .410 buck for my judge and also some of the Winchester PDX1 Defender ammo. I also have 45 rounds of 12 ga. 000 buck on hand.
I tend to buy ammo when I can find a good deal on it. I recently was able to pick up 3 cases of Federal Wing Shok (predecessor to Prairie Storms) 3" #5's for $100.00 / case! Great load for the wiley Dakota Roosters in the fall. At $10 / box it was a great deal as I can't find this shell any longer except on line an then they are $30+ / box.
I generally carry more rifle and handgun ammo than shotgun. I have .22LR as it is still the most cost effective way to shoot alot. I have quite a bit of .223 /5.56 and 45acp and 9mm.