Task: Text to Code
Input (English): Step 1: find the apt_id of Apartment_Bookings table.
Step 2: find the number of rows in Apartments table whose Apartments's apt_id not one of the results of step 1
Output (English): SELECT Count ( * ) FROM Apartments AS T1 WHERE T1.apt_id NOT IN ( SELECT T2.apt_id FROM Apartment_Bookings AS T2 )