Haven't actually bought this as I feel too preoccupied with other things, but The Pokémon Company have releases this statement about the first 4 days of sales:
Global sales of Pokémon Pokopia for the Nintendo Switch 2 system have surpassed 2.2 million units (including 1 million units sold in Japan) in the first four days since its worldwide launch on March 5, 2026*.
Physical estimates in Japan are around 500-700k, which makes me think it's probably around the lower end of that estimate (which is still huge and possibly the highest first week sales for a Pokémon spin-off in Japan), with digital ratio around half for Nintendo in Japan. The Famitsu sales numbers are released tomorrow, but are still essentially estimates that often require readjusting over time.