There are estimates how common the disorder is among adults and children. Depending on how those are established, 2.5% to 5% seems like a probable range.

https://www.forbes.com/health/mind/adhd-statistics/

https://www.nimh.nih.gov/health/statistics/attention-deficit-hyperactivity-disorder-adhd

https://www.sciencedirect.com/science/article/abs/pii/S0890856722001903