user: Question: How many incumbents were first elected in 1984? 
Columns: ['District', 'Incumbent', 'Party', 'First elected', 'Results', 'Candidates']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Pennsylvania 1', 'Bob Brady', 'Democratic', 'May 19, 1998 (special)', 'Re-elected', 'Bob Brady (D) 81% William Harrison (R) 17%']
SQL:

assistant: SELECT COUNT("Incumbent") FROM "Table" WHERE "First elected" = '1984';