Write SQL query to solve given problem: How many planes of Spirit Air Lines took off on 2018/8/7?. Keep the solution inside sql tag ```sql [SQL-Query] ```
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'