That is probably true in general, but I've seen the occasional exception, such as the IAC Rose described by Budd & Hansen (1896).
http://bulbnrose.x10.mx/Roses/breeding/ ... n1896.html
I. A. C. ROSE.
This is our No. 1 of the many seedlings produced by fertilizing the Russian Rosa rugosa with pollen of the Gen. Jacquiminot. The bush is a rampant grower, now four and one [half] feet in height with many branches. It is less thorny and its leaves are thicker, more leathery, and glossy than those of either parent. So far it seems a model of health, and able to endure the extremes of summer heat and drouth.
The first flowers opened July 22nd. The flowers average larger than those of Gen. Jacquiminot, are much more perfectly double, containing as high as sixty‑six petals of a beautiful dark crimson color much like the Russian Rosa rugosa, and delightfully fragrant. As the mother has but five petals and the male parent but about forty, the perfect doubling of the hybrid is remarkable. Possibly it has bred back to some ancestor of the Gen. Jacquiminot.
One thing that comes to mind is that various Rosa species differ in their numbers of stamens, which are transformed into petals by "doubling". For example, according to Erlanson (1934), Rosa woodsii averages 65 stamens. R. blanda, on the other hand, has from 85 to 140. If specimens of both species were crossed with a China rose, with cross is likely to have offspring with more petals?
Then there is R. palustris with more than 200 stamens per flower, if you really want to pack in the petals.