Prevalence is the proportion of people in a population who have a particular condition at a given time, usually shown as a percentage.
Prevalence differs from incidence, which counts new cases over a period. A condition can have high prevalence because it lasts a lifetime, even if new diagnoses are rare.
Prevalence figures for ADHD and autism vary between studies, countries and decades, depending on how people were sampled and which criteria were used. A figure based on diagnosed cases is lower than the true figure whenever many people are never assessed.
We don't quote prevalence figures on this site unless we can name the study they came from. A number without a source tells you very little.