I try to have 50 12 Ga Buckshot shells and at least 5 Slugs.
For my 20 Ga SxS, I am still building-up. I finally found some #3 20 Ga Buckshot loads and was able to get 5, 5 shot boxes and a 5 round box of 20 ga Slugs. I am going a little slow building that supply of #3 Buck up. but the way my sight is going bad, I wnt more!
I figgure 50 Buckshot loads is reasonable for each Gauge.
Like was said above, I don't plan on an extended Shotgun fight! If I ever need it to work,and if I can hit with it, I figgure one plus one "more" to be sure is enough per bad guy!