A 'single' rose flower normally has five petals. This applies to all roses - climbers, ramblers and bush roses. Five, is the number of petals that this family group (Rosacea) has.
However! As plant breeders have come to realise our absolute fascination with this group of plants, the rose has developed from the original wild hedgerow 'Dog Rose' into semi double and fully double blooms which are the norm today.
The 'single' rose - as with its nearest cousin the 'semi-double' rose, has petals that open out wide, showing off the stamens. (For insects/bees to find, in order for them to pollinate the flowers.
It is interesting that these singles and semi-double roses are coming back into fashion - especially with the dwarf or miniature patio roses. One of the most outstanding examples of a fine 'single' rose, is R. 'EyePaint'.
The highly bred/hybridised floribunda and HT roses which are the mainstay of the rose population, can have as many as a hundred or so petals. (Have not counted them!). These are true 'doubles, and in most cases there are no stamens to be found within the tight cluster of flowers, as even the stamens in some, have become petals in a way!