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Definitely way less.
For starters the release of the Wii would've cut off it's legs. Even though the Wii is backwards-compatible and the GC version would have been playable, 'nobody' would have wanted to play a last-gen game. Then, the Zelda curse really is a thing, so it likely wouldn't have sold more than Wind Waker. It was the Zelda game 'everybody' wanted at the time though so I wouldn't have gone as low as Majora's Mask even with the installbase. I'd vote 3.5m-4.5m, with around 4.0m being it.
Releasing it on Wii besides GC was just good business, as releasing Zelda U also on NX would be to.
Also interesting would be the question what it would have done to Skyward Sword's sales had Twilight Princess been GC exclusive, which would then have been the first and only Zelda Wii game...
