How many planes does Southwest Airlines Co. have?
planes refers to TAIL_NUM; Southwest Airlines Co. refers to Description = 'Southwest Airlines Co.: WN';
SELECT COUNT(T3.TAIL_NUM) FROM ( SELECT T1.TAIL_NUM FROM Airlines AS T1 INNER JOIN `Air Carriers` AS T2 ON T1.OP_CARRIER_AIRLINE_ID = T2.Code WHERE T2.Description = 'Southwest Airlines Co.: WN' GROUP BY T1.TAIL_NUM ) T3