user: Question: How many different production codes does the episode originally aired on October 18, 1984 have?
Columns: ['No. in series', 'Title', 'Directed by', 'Written by', 'Original air date', 'Production code']
Types: ['real', 'text', 'text', 'text', 'text', 'real']
Sample row: [1.0, '" Pilot " "Theo\'s Economic Lesson"', 'Jay Sandrich', 'Ed. Weinberger and Michael Leeson', 'September 20, 1984', 101.0]
SQL:

assistant: SELECT COUNT("Production code") FROM "Table" WHERE "Original air date" = 'October 18, 1984';