How many cities have hosted events?
select count(distinct city) from city as t1 join hosting_city as t2 on t1.city_id = t2.host_city
For each city, how many events has it hosted?
select t1.city, count(*) from city as t1 join hosting_city as t2 on t1.city_id = t2.host_city group by t2.host_city
Which city has hosted the most events?
select t1.city from city as t1 join hosting_city as t2 on t1.city_id = t2.host_city group by t2.host_city order by count(*) desc limit 1