I think the real key is trying to be realistic. This depends entirely on where you hunt and how you hunt.

Where I hunt in VA, you'll see fairly big rafts of teal, but the big ducks will tend to be in groups of 4-10. So I like to keep my spreads of big ducks just like so. I mix in mallards and blacks with an odd spring in for color. Sometimes I'll have a few teal off to the side, or some wood ducks on the far reaches, almost hidden. As the season gets later, I'll use small bunches, such as two drake mallards and a hen... as birds are starting to pair off into mating couples, and competition can get fierce for hens.

Most importantly, you have to play the wind. Make it easy and conveinent for the birds to land. They need plenty of space. Its some sort of duck taboo to fly over other ducks when coming in to land.... so leave the hole just off to the side so that everyone can have a shot, and the incoming birds will not be staring directly at you. Also keep in mind (if you hunt creeks like me) that ducks would rather fly over water to land then ground... so if they can fly up or down the creek to light... they will do it.

One other thing... don't group them so close together... because often ducks only bunch up when they are nervous or scared.