Pleiomerous: Having more than the usual number of parts in a floral whorl; e.g. roses (Rosa) that have a proliferation of petals.*
In nature, roses (genus Rosa) have flowers with only 5 petals. Many garden roses, such as the one in this picture, have been selectively bred to have many extra petals.
*Pell, S.K., Angell, B. (2016). A botanist’s vocabulary: 1300 terms explained and illustrated. Portland: Timber Press.