To be honest I have some problems with these labels. From what I can tell, we are doing attachment parenting, without having ever heard of it, by just doing what feels right and is easy. It's a natural thing to do.
So why attach a label to it? It turns it into some sort of movement, means you have to do it "Just the right way", as described in the books. For example we have been more or less co-sleeping for a year now, but it isn't working anymore, but when asking how to stop we get several responses telling us we shouldn't, because Attachment Parenting is so great and you have to follow it by the book. It becomes dogma. Therefore I think it's a bad idea to do Attachment Parenting. It is a good idea to follow the practices in Attachment Parenting, but as soon as you attach a label to it it becomes a fixed set of things to do, that you then may end up forcing yourself or your child to do even when it's not right for you, but because it's included in the label. Sure, most people won't do that, but do they then continue to use the label or not?
And that brings me to the actual question: Statistics like this will be very misleading, as many more people do attachment parenting than any statistics would show, because they don't know what it is. Many probably know what it is, say they follow it, but don't follow it by the book. Should they then be counted at doing it or not? How much of it do you need to do to classify as attachment parenting?
So the answer probably is: Everyone does it, more or less. Some a lot, some very little. Where you put the line to say "this is attachment parenting and that is not" becomes arbitrary, and therefore any percentage would be arbitrary as well.