Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
How many doubles champions were there for the Serbia f6 Futures tournament?

### Inputs:
table_name: table, columns: ['Tournament', 'Date', 'City', 'Country', 'Surface', 'Singles champions', 'Doubles champions']

### Response:
SELECT COUNT Doubles champions FROM table WHERE Tournament = Serbia F6 Futures