How many planes of Spirit Air Lines took off on 2018/8/7?
Spirit Air Lines refers to Description = 'Spirit Air Lines: NK'; on 2018/8/7 refers to FL_DATE = '2018/8/7';
SELECT COUNT(T2.Code) FROM Airlines AS T1 INNER JOIN `Air Carriers` AS T2 ON T1.OP_CARRIER_AIRLINE_ID = T2.Code WHERE T1.FL_DATE = '2018/8/7' AND T2.Description = 'Spirit Air Lines: NK'