When someone refers to a rose as a single, they don't mean it has only one petal. Just that it has a smaller number of petals than other roses -- from five to twelve. Unlike fuller blossoms, single roses often close up at night.
[From Miniature Roses: Their Care and Cultivation, by Sean McCann, p. 132-3:] It was Ralph Moore who began the interest in single-petalled miniatures when he introduced 'Simplex' in 1961; the five petals are white with a golden centre... Most rose shows have a class for the single-petalled miniatures, but they are difficult to show as the main attraction to the judge's eye will be freshness, and this fresh look is hard to maintain. If a room is too hot they can just turn over and disintegrate; if it is too cool they shiver and close up before judging begins. Showing single miniatures is an art in itself. However, their beauty, if fleeting, stays in the memory for a long time.
[From Roses by Susan Bales, p. 47:] 4 to 7 petals in a single row.
[From The American Horticultural Society Encyclopedia of Gardening, Christopher Brickell, Editor-in-Chief, p. 115:] Single (4-7 petals)