What is the total number of orders made by customers in United States?
orders refer to o_orderkey; the United States is the name of the nation which refers to n_name = 'UNITED STATES';
SELECT COUNT(T1.o_orderkey) FROM orders AS T1 INNER JOIN customer AS T2 ON T1.o_custkey = T2.c_custkey INNER JOIN nation AS T3 ON T2.c_nationkey = T3.n_nationkey WHERE T3.n_name = 'UNITED STATES'