user: Question: The Mackinnon Stakes races took place on how many venues?
Columns: ['Result', 'Date', 'Race', 'Venue', 'Group', 'Distance', 'Weight (kg)', 'Jockey', 'Winner/2nd']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Won', '28 Aug 2010', 'Memsie Stakes', 'Caulfield', 'G2', '1400 m', '58.5', 'S. Arnold', '2nd - Whobegotyou']
SQL:

assistant: SELECT COUNT("Venue") FROM "Table" WHERE "Race" = 'Mackinnon Stakes';