A sample is the group of people who take part in a study, chosen from a larger population so the results can tell us something about that population.
Results only apply as far as the sample resembles the people you want to know about. A study of 8-year-old boys in one US city tells you little about women in their forties in Cork.
Sample size matters too. Small samples produce results that vary a lot by chance. How people were recruited matters just as much. A sample drawn from people already diagnosed misses everyone who was never assessed, which for ADHD and autism includes many women, older adults and people who mask.
When we publish figures from our own screeners, we will say how many people they are based on and who those people were.