Maybe there isn’t a physical number.
Maybe the scale is a non-integer value
Maybe your window is rotated at 20 degrees so that the physical pixels don’t even line up with the virtual ones.
Maybe the resolution is so high that a human can’t even see the pixels.
Maybe the display gets scaled in hardware so that the physical pixels it reports don’t actually correspond to the ones on the screen (this is common in phones)
I get so tired of these “physical pixel” questions - they always turn out to be someone trying to make some graphics fit the pixels on the display they’re currently staring at, without considering where else it might be used.
Physical pixels mattered in the 1990s when VGA was still a thing, but people have all kinds of displays now, you need to think about your graphics in a more abstract way.