I know that at least half of people who have ADHD have comorbidities and I’ve come across some higher estimates than this. There’s lots of information about this on http://www.additudemag.com – I find this a great website for info on ADHD with some good webinars. This is quite a recent article:
I hope that is helpful. From a bit of philosophical perspective I think there is mileage in us acknowledging that ADHD, Autism, DLD, dyslexia and many other neurodivergences are only separate conditions because we have described a number of characteristics and given them a name. The fact that so often more than one diagnosis fits perhaps gives the real picture of human neurodiversity. Differential diagnosis can sometimes be very difficult, but maybe because we are looking for separations that don’t actually exist in reality. Not sure if I’m making any sense, it’s just something I have been thinking about recently!