user: Question: how many times is the fuel propulsion is CNG?
Columns: ['Order Year', 'Manufacturer', 'Model', 'Fleet Series (Quantity)', 'Powertrain (Engine/Transmission)', 'Fuel Propulsion']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['1992-93', 'Gillig', 'Phantom (High Floor)', '444-464 (21)', 'DD S50EGR Allison WB-400R', 'Diesel']
SQL:

assistant: SELECT COUNT("Fleet Series (Quantity)") FROM "Table" WHERE "Fuel Propulsion" = 'CNG';