Amy!
See you are a Michigander. There are several of us in this forum.
Is it your goal to have a large bunch of one color iris in a particular space? The dwarf iris are fast growers and will make clumps quickly, but if you intend to fill a space then you may wish to get more than one as Tom suggested above.
If you want splashes of different colors then plant each different named iris at least one foot apart. For example, let's say you have the following and want to have them fill an edging:
When you first plant them, it will look like a foot is pretty far apart, but they will fill that gap in no time as they grow and increase.
