Count the number of participants for each event.
select event_id,count(*) from participants_in_events group by event_id
Among those, which event id has the least participants?
select event_id from participants_in_events group by event_id order by count(*) asc limit 1
What about the one that has the mose participants?
select event_id from participants_in_events group by event_id order by count(*) desc limit 1