The figures for how many of 'these dogs' (difficult in the first place as this covers a wide range of crossbreeds) are around are WAY out.
Stats for how many dogs, what breed etc, come from two main sources:
1/Kennel Club breed registrations - not applicable.
2/Insurance companies - highly unreliable when so so many will not be insured as individuals for vet bills (they may be covered as non-specific animals within a household under other policies).
They're also not new, they've been around a good 20 years, though the numbers have increased massively in the last few years, but then this follows the trend of dog ownership in general.
In the course of my work, I have had cause to report to SS on just one occasion - that is across almost 20 years of going into peoples homes, training classes and latterly, working remotely.
That case was not xl bullies. It was two akitas and a near pedigree type (tiny!) staffy.
Even then, it was the ignorance and risk taking of the owners that caused me to report rather than the breed of dog.
I've worked with xl bullies of all shapes, sizes and genetic origin - some I'd say were no more likely to attack than any other dog/breed, and some an absolute fucking liability and the difference was always the attitude and behaviour of the owner far more than the behaviour of the dog.
All of us who work with dogs responsibly, will contact SS if we're concerned about a situation. Unfortunately, the really scary people will be the least likely to contact a trainer/behaviourist in the first place.
These days that is even truer, as people think they can get all their dog training and handling information from TV shows or TikTok - and for the last three years or so, the 'wealth' of horrific abusive training methods displayed on Tiktok and similar has sky-rocketed.
It just isn't as simple as banning this, automatically assuming that, if life were that simple we'd all be living in Utopia... there'd be world peace, no poverty, no drug problems etc etc. It is complicated, because humans make it so!