Splatoon 1 on the Wii U is at 4.8 million as of the end of March. https://www.nintendo.co.jp/ir/en/finance/software/wiiu.html
So, it pretty much already hit that 5-6 million mark on the Wii U.
If it does double that amount, than we're looking at 10-12 million for 2 on the Switch.
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