How many times did each party use services?
select count(*) from parties as t1 join party_services as t2 on t1.party_id = t2.customer_id group by t1.party_id
Which party used party services the most number of times?
select * from parties as t1 join party_services as t2 on t1.party_id = t2.customer_id group by t1.party_id order by count(*) desc limit 1
What is their email?
select t1.party_email from parties as t1 join party_services as t2 on t1.party_id = t2.customer_id group by t1.party_id order by count(*) desc limit 1