In short, ammunition is divided into two categories, which are hunting ammunition and defense ammunition. Both types of ammunition are very essential for SHTF. For defense, you must store a minimum of 500 rounds of defensive ammunition for your shotgun, approximately 2000 rounds for your rifle, and finally 1000 rounds for your gun. Let's take those numbers and lower them to 1000 rounds for bullets, small game and pellets.
That's still equal to 3000 shells for the shotgun. Store ammunition in their original packaging and stack them off the ground on a shelf or inside plastic ammunition boxes where they are protected from water. If you store it in an area, such as a garage or basement where moisture is an issue, you can also try using a dehumidifier or, when you store it inside a closet, refillable products that absorb moisture. If you use them, be sure to check them often and change them as needed.
However, if you use the shotgun to hunt ducks, geese and pigeons, you will need a lot more ammunition around the house. When calculating how much ammunition you should have in the house, don't forget to count the training time and ammunition shortage in gun stores. The principle behind how much ammunition you should keep for shotguns is similar to that of a hunting rifle, but keep in mind the type of shot you use. Maybe even increase those numbers to around 1000 shotgun shells; have two of these shotguns with 1000 shells between them.
If your shotgun is your primary local defense weapon, you'll need more ammunition than you would have on hand.