user: Question: What is the highest sample size administered on October 18?
Columns: ['Poll Source', 'Date(s) administered', 'Sample size', 'Margin of Error', 'Mel Martinez (R)', 'Betty Castor (D)']
Types: ['text', 'text', 'real', 'text', 'text', 'text']
Sample row: ['Zogby International', 'October 31', 600.0, '± 4.0%', '46%', '46%']
SQL:

assistant: SELECT MAX("Sample size") FROM "Table" WHERE "Date(s) administered" = 'October 18';