600-700k. Better than the Lite's debut, but nothing monstrous.
Considering the biggest non-holiday week for the Switch (according to VGC, at least) was a bit over 1M, I doubt we'll see a repeat of that. Considering that A) there's a potential for shortages and B) there's the potential for most existing Switch owners to not view it as much of an upgrade, the odds of it setting some sort of new record are probably nil. It should still do very well, at least in the short term. Our first big indication on how well it actually does will be when Famitsu sales drop in 8 days. For reference, the best debut for a hardware revision in Japan on record was the New 3DS, which by itself sold ~235k in its first week (the other extant 3DS models sold ~27k combined), while the runners-up are the 3DS XL at ~187k, the Switch Lite at ~178k, and the DSi at ~172k.