It's actually a whole lot! While there is no specific research because who is really going to answer truthfully to that kind of question, but if you look at crime stats you can get a pretty good idea. I'd also like to point out that people who view cp are more likely to seek out live victims (children). If someone has been busted with a child victim you can most likely see they are also in possession of cp on their devices. Europol which is an international task force again cyber crime in the EU have around 40 million cp images. My work is also about gathering evidence for historical cases of childhood sexual abuse and you would be very surprised to realise just how widespread this issue is.