big key is scouting, find the field that birds are using, and you wont need very many decoys. all I use is about 2 doz. birds will use pretty much the same spot in the field or close to it. take note of that and you should be fine with 2 doz, I have 1 doz silos, and 1doz shells, and 4 fullbodys. But some swear by a BIG spread, I cant afford it, so I just spend more time doing my "homework". For shells, I shoot 3 1/2" 2 hevishot, and BB, sometimes BBB, depends on if I am pass shooting, or decoying. how many shells? That depends on your shooting. If you are skybusting, you could easily go through a box before you get a goose. just shoot at what you know you can hit, put your bead right on the nose of the goose.
Good luck! Let me know if you wanna shoot anything up this way.