Generally 2-3 doz suffices. You can add Mallards, Blacks, Widgeon, Teal, Pintails, and other species relative to your area. You can also add coots and or goose decoys for confidence decoys but having the goose decoys adds to bagging the odd honker as well. A spread of 3-9 will generally work fine.
You can also add some Ringbills, Bluebills, and also a few (1 1/2 doz ) GoldenEyes as the season progresses.
Most of this comes with experience as to, how many of each you set, the size of the body of water, the time of season, and how many birds are in your area.
Mind you......this is only a tip of the iceberg so to speak.
Give me some more info and I'll see if I can help you out in the future.