How many pieces of the 8 ft. Corrugated Galvanized Steel Utility-Gauge Roof Panel are needed for a 20 ft. x 9 ft. roof?

you would need 10 panels

When used for siding panels you get 24" width coverage so you would need 10 panels to cover 20' if installing the panels vertically. The panels are stocked in 8', 10' and 12' lengths.