user: Question: What was the number of episodes whose production code is 7ABB21?
Columns: ['№', '#', 'Title', 'Directed by', 'Written by', 'Original air date', 'Production code', 'U.S. viewers (millions)']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [123.0, 1.0, '" Lessons "', 'David Solomon', 'Joss Whedon', 'September24,2002', '7ABB01', '5.00']
SQL:

assistant: SELECT COUNT("#") FROM "Table" WHERE "Production code" = '7ABB21';