Saturation is when you are not selling a console to potential buyers because they already have one.

Japan has a population of around 125.8 million, and Switch has sold 23.5 million there in total. That's a 18.68% ratio.

If, let's say, only 40% of japanese people are potential buyers, switch saturation would be close to 50 million consoles (plus double dip and broken switches).

Yeah, Switch is not yet at saturation.
That's more of a point where sales start to seriously diminish as there are few people left in the target audience without one, I see no sign that the Switch has reached that stage yet. Being passed the peak doesn't mean it's saturated.

Saturation is more like where PS4 was in 2020.

Before saturation really kicks in Nintendo can delay it with a price cut. I think in Japan saturation will start earlier than in other regions, especially Europe. If they don't make a price cut I can see saturation setting in sometime next year, likely later.
With a price cut though the Switch can definitely outsell the DS.

It's harder to discern saturation as we've never had a dedicated hybrid platform before, it's selling in two parallel markets so even with overlap the patterns of such a platform's life cycle may very well be significantly different than anything dedicated to just one market.

Yeah, don't expect any price cuts. The console manufacturers are not about that kind of strategy anymore.

Yeah that's what I define saturation with but is the decline in sales because of saturation or less stock available for Japan? If they sell less because their is less available we may not call it saturation yet.

Correct, Switch OLED is harder to find in Japan and Nintendo was saving some stock for Arceus release.

Switch will probably start to hit saturation point next year I expect, assuming it's not cut short prematurely.