Super Mario Odyssey did 9.07m when Switch was at 14.86m.
SSBU will likely beat SMO because Switch will have already exceeded 30m, but 15m in one month sounds like the result of an ecstasy trip. What kind of regional breakdown for sell-through do you have in mind, tbone? 3m for Japan, 4m for the USA, 3m for Europe, 1m for the rest? That's 11m and a lot of loaded retailers to make that 15m shipped.
I am expecting a breakdown in the area of 2.5m for Japan, 3.5m for the USA, 2m for Europe and 500k for the rest, so 8.5m sell-through and shipments between 10-12m, but more likely above 11m than below 11m.