HMF uses the following approximate petal counts to categorize the fullness of rose blooms:
Single Roses have 4-8 petals, Semi-double Roses have 9-16 petals, Double Roses have 17-25 petals, Full Roses have 26-40 petals, and Very Full Roses have more than 40 petals.
It may be useful to keep in mind that some rosarians mean different things when using some of these terms. Some writers may class as "double" any rose that is not single, for example.
Also a rose bloom's fullness, size and shape can vary according to seasons and growing conditions.