CREATE TABLE table_15849 ("Order Year" TEXT, "Manufacturer" TEXT, "Model" TEXT, "Fleet Series  ( Quantity ) " TEXT, "Powertrain  ( Engine/Transmission ) " TEXT, "Fuel Propulsion" TEXT)

how many times is the fleet series (quantity) is 468-473 (6)?
SELECT COUNT("Order Year") FROM table_15849 WHERE "Fleet Series (Quantity)" = '468-473 (6)'
SELECT COUNT("Order Year") FROM "table_15849" WHERE "Fleet Series (Quantity)" = '468-473 (6)'