user: Question: What is the number of candidates for Pennsylvania 21
Columns: ['District', 'Incumbent', 'Party', 'First elected', 'Result', 'Candidates']
Types: ['text', 'text', 'text', 'real', 'text', 'text']
Sample row: ['Pennsylvania 2', 'William T. Granahan', 'Democratic', 1948.0, 'Re-elected', 'William T. Granahan (D) 57.0% Max Slepin (R) 43.0%']
SQL:

assistant: SELECT COUNT("Candidates") FROM "Table" WHERE "District" = 'Pennsylvania 21';