user: Question: How many total units were built of Model DS-4-4-660 with a B-B wheel arrangement and a PRR Class of BS6?
Columns: ['PRR Class', 'Builder’s Model', 'Build date', 'Total produced', 'Wheel arrangement', 'Service', 'Power output']
Types: ['text', 'text', 'text', 'real', 'text', 'text', 'text']
Sample row: ['BS6', 'VO-660', '1942–1945', 12.0, 'B-B', 'Switcher', 'hp (kW)']
SQL:

assistant: SELECT "Total produced" FROM "Table" WHERE "Wheel arrangement" = 'B-B' AND "PRR Class" = 'BS6' AND "Builder’s Model" = 'DS-4-4-660';