Du behøver ikke den underforespørgsel, der finder den maksimale værdi.
I stedet,; du skal bare bruge den første række efter at have bestilt rækkerne:
select * from (
select
membership.mem_desc,
membership.mem_max_rentals,
membership_history.mem_type,
count(membership_history.MEM_TYPE) as membership_count
from membership_history
JOIN membership ON membership.mem_type = membership_history.mem_type
group by (membership_history.mem_type,membership.mem_desc,membership.mem_max_rentals)
ORDER BY 4 DESC -- Added this line
) g
WHERE ROWNUM = 1. -- Added this line