What is the region with the most customers?
region refers to r_name; the most customers refers to max(count(c_custkey))
SELECT T.r_name FROM ( SELECT T3.r_name, COUNT(T2.c_custkey) AS num FROM nation AS T1 INNER JOIN customer AS T2 ON T1.n_nationkey = T2.c_nationkey INNER JOIN region AS T3 ON T1.n_regionkey = T3.r_regionkey GROUP BY T3.r_name ) AS T ORDER BY T.num DESC LIMIT 1