user: Question: In Round 6, how many winning drivers were there?
Columns: ['Rnd', 'Race Name', 'Pole position', 'Fastest lap', 'Winning driver', 'Winning team', 'Report']
Types: ['real', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [1.0, 'Marlboro Grand Prix of Miami Presented by Toyota', 'Gil de Ferran', 'Juan Pablo Montoya', 'Max Papis', 'Team Rahal', 'Report']
SQL:

assistant: SELECT COUNT("Winning driver") FROM "Table" WHERE "Rnd" = 6.0;