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Every game would end in a timeout or a instant 6/5-0 sweep if full 6v6.
Official VGC already only allows 10 minutes of play time and a full turn out tournament can still take days to finish since every game is best 2 out of 3.
The default in game setup is generally 3v3 which is total ass as everything turns into a boring sweep. My guess is a official singles if it ever happened and you wanted to get a balance would be 4v4.
It'd never happen though since Doubles simply allows for a lot more interesting plays whilst Singles always devolves into a switch fest and second guessing.