A TGV Duplex formation has a maximum of 10 cars: two power cars and eight passenger cars.
The TGV Duplex is a French high-speed train developed by Alstom for SNCF. Its passenger coaches have two levels, allowing the train to carry more people without making the vehicles wider or exceeding railway loading-gauge limits. A standard set is about 200 metres long and can carry roughly 500 passengers, depending on the interior layout.
The ten-car figure includes the two streamlined power cars at the ends. The eight passenger cars between them are the double-deck coaches that give the Duplex its name. Counting only passenger coaches would therefore produce eight, but counting the complete train formation produces ten.
A common source of confusion is coupled operation. TGV Duplex sets can run as two units working together, creating a train of 20 cars. That is not the normal formation of one Duplex set, however. The question asks about a single formation, for which 10 is the standard canonical answer.