In which country do most of the customers come from?
country refers to n_name; most of the customers refer to MAX(COUNT(c_custkey));
SELECT T.n_name FROM ( SELECT T2.n_name, COUNT(T1.c_custkey) AS num FROM customer AS T1 INNER JOIN nation AS T2 ON T1.c_nationkey = T2.n_nationkey GROUP BY T2.n_name ) AS T ORDER BY T.num DESC LIMIT 1