Wii U can use two GameCube controller adapters
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Wii U 4 Port Gamecube Controller Adapter
Amazon.com: Gamecube Controller Adapter for Switch/Switch OLED.Gamecube Adapter Compatible with PC Wii U.Adapter with 4 Ports 6 FT(1.8M) Cable Support Turbo and Vibration Features Super Smash Bros : Video Games
Smash Wii U GameCube Controller and Adapter Unboxing - YouTube
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GameCube Controller Adapter for Wii U and PC USB Two ports
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Confirmed: The Wii U GameCube Controller Adapter will not work on DDR, Active Life, or Cyberbike | Gaming.Fit
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Mayflash 4 Ports For Gamecube Controllers Adapter Converter For WIIU S
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gamecube Controller Adapter (For Switch/WiiU/PC) – YCCTEAM
