I just buy one of each, and plant them at least 2 and a half feet apart. I like to plant companion plants with them that bloom along with them, such as chives, giant allium, spirea, foxglove, and peonies. But I also like to plant other plants that will give color in the garden both before and after the iris bloom. FOR THE "BEFORE" I have such things as daffodils, tulips, grape hyacinths, bleeding heart, brunerra, tiarella, helleborus, squills, chionodoxa, and crocuses, and forsythia.
For the AFTER, I have daylilies, ferns, hosta (deer candy), asters, chrysanthemums, siberian iris, Louisiana iris, atilbe, roses, salvia, russian sage, yarrow, true geranium, coreopsis, oriental poppies, and deer-resistant annuals such as sweet allysum, cleome, cosmos, and shirley poppies. Also caryopteris, and monkshood. And for winter interest (winter berries), winterberry holly, nandina, and skimmia japonica.
