ooof prediction time, so in terms of points its likely gonna come under splatoon 2. We know splatoon 2 opened to ~675,000. Now this woulda been the defining factor if pokemon lets go didn't come out. Pokemon lets go opened to 661,240 with a point score of 279. So that was a ratio of 2.37 extrapolated. If we applied that to this score it would be insane we would be looking at ~2133032 MILLION UNITS SOLD!
so to narrow this a little further we have to look at the last iteration games that came out and see their attach ratios.
With that said smash for 3ds opened with 958,317 to a HW base of ~16m. Due to the 3ds's popularity guaranteed that multiple 3d's belong to the same users. so just gonna average that out to ~15.5m. This was back in 2014 so 3 and a half years after the initial release. We can deduce that by that point initial users likely would have stopped playing the handheld or it simply broke. lastly factoring in that this was the first smash iteration which is a fighter on the 3ds that wasn't known for the genre we can bump that down a heck of a lot more to ~4m (my guess). lol with that possibly made up number we get an attach ratio of 23% of those who may be into the genre buying this game. Extrapolating that for switch we get 5.2m*.23 = 1.2m.
LITT LOL SO I HAVE MY NUMBERS. Anything between 1.2m-2.1m units.