Today it was revealed via Twitter that the Wii U console can allegedly support two Gamecube Adapters. This would make sense seeing how the Wii U version of Super Smash Bros. can support up to 8 Player simultaneous matches.
This may become an issue for those who opt for the Digital Download. As each adapter requires two USB slots, using two would mean that there is no space for an external hard drive. Then again, you could always install Smash Bros. on the internal drive on its own.