user: Question: What is the sum of Number of fixtures when the rounds shows Quarter-finals?
Columns: ['Round', 'Main date', 'Number of fixtures', 'Clubs', 'New entries this round']
Types: ['text', 'text', 'real', 'text', 'text']
Sample row: ['Preliminary round', '29 August 2007', 16.0, '48 → 32', 'none']
SQL:

assistant: SELECT SUM("Number of fixtures") FROM "Table" WHERE "Round" = 'Quarter-finals';