It is as variable as Safrano, CybeRose. The higher the temperature, the fewer the petals.
In hot weather, not only do the blooms have fewer petals, the petals have less substance and lustre. The blooms tend to be smaller and pass from bud to spent bloom very quickly.
In cooler weather it all happens more slowly. The blooms tend to be larger, they have many more petals and the petals are more substantial and lustrous.
In our climate (hot summers, mild winters) both Safrano and Isabella Sprunt produce their fullest and loveliest blooms in winter