Peonies are not plants that spread themselves around the garden to create new plants. You need to lift and divide existing clumps of peonies to increase their numbers. Another option to get more plants is to harvest peony seeds and sow them undercover to grow new ones. This means you need to refrain from deadheading peonies completely, leaving one or two blooms for the purpose of collecting seeds. The alternative is to buy new plants, for example take a look at the range of peonies available at Nature Hills.