user: Question: How many doubles champions were there for the Serbia F6 Futures tournament?
Columns: ['Tournament', 'Date', 'City', 'Country', 'Surface', 'Singles champions', 'Doubles champions']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Germany F1 Futures', 'January 11', 'Schwieberdingen', 'Germany', 'Carpet (i)', 'Jesse Huta Galung', 'Jesse Huta Galung / Miliaan Niesten']
SQL:

assistant: SELECT COUNT("Doubles champions") FROM "Table" WHERE "Tournament" = 'Serbia F6 Futures';