The more water you are duck hunting around, the more decoys you should use. The point of these decoys is to make the ducks you are trying to shoot feel as comfortable as possible, so they will land. The more you make it seem like there are a lot of ducks hanging out on the water, the less these ducks you are hunting will think there is danger lurking nearby. However, you don't want to overload a particular area of water, as this will scare living ducks off. For a small pond of water, use up to 12 decoys. For a larger lake, use a couple dozen decoys, spaced apart by about 50 to 100 feet, so that you can shoot the ducks that land in the water between them.
Duck Decoys Are Like Fishing Lures
If you have ever done any fishing, and most duck hunters have at some point in their lives, you know that using the same fishing lure every time you go out will not always yield good results. This is because fish are biting different lures throughout the year and also learn that certain lures are not food but things that will get them caught. Ducks are very similar in this sense, as different decoys appeal to different ducks at different times of the year. So have a number of different duck decoys that can be switched during the duck hunting season each year, or even each day that you are out hunting.
Light but Not Reflective
There are many different types of duck decoys out there and the main thing you should look for in how they are constructed is that they are light to carry and don't have reflective surfaces. As you are going to have to be trudging out into water and swampland with the decoys in hand, the lighter these decoys are the better, so that you aren't wearing yourself out and scaring off all the ducks in the area. Also, make sure the surfaces of these duck decoys aren't reflective in any way, as any glint from these decoys will usually scare off ducks that would otherwise land on the water. Try to get duck decoys made out of a number of different materials, ranging from wood to durable foam to thicker plastic. These subtle differences will appeal to different ducks at different times of year.