user: Question: what of the total number of date where grand prix is Portuguese Grand Prix
Columns: ['Round', 'Grand Prix', 'Date', 'Location', 'Pole Position', 'Fastest Lap', 'Winning Driver', 'Winning Constructor', 'Report']
Types: ['real', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [1.0, 'South African Grand Prix', '1 March', 'Kyalami', 'Nigel Mansell', 'Nigel Mansell', 'Nigel Mansell', 'Williams - Renault', 'Report']
SQL:

assistant: SELECT COUNT("Date") FROM "Table" WHERE "Grand Prix" = 'Portuguese Grand Prix';