Assume olympics_athletes_events(id, games, event, ...), with potentially several rows per athlete per games because an athlete can enter several events.
WITH participation AS (
SELECT games, COUNT(DISTINCT id) AS athletes_count
FROM olympics_athletes_events
GROUP BY games
)
SELECT games, athletes_count
FROM participation
WHERE athletes_count = (
SELECT MAX(athletes_count) FROM participation
)
ORDER BY games;
For (Athlete 1, Games A, Event X), (Athlete 1, Games A, Event Y), and (Athlete 2, Games A, Event X), there are three event rows but two athletes.
If Games A and Games B each have two athletes, both should survive. ORDER BY athletes_count DESC LIMIT 1 loses a tied winner. Ranking the aggregated counts and retaining rank 1 is another valid approach, but the maximum comparison is sufficient for this question.