user: Question: How many dates have Silverstone circuit
Columns: ['#', 'Race Name', 'Circuit', 'Date', 'Winning driver', 'Constructor', 'Report']
Types: ['real', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [1.0, 'IV Lombank Trophy', 'Snetterton', '30 March', 'Graham Hill', 'BRM', 'Report']
SQL:

assistant: SELECT COUNT("Date") FROM "Table" WHERE "Circuit" = 'Silverstone';