Yes, I am asking: all else equal, would more NATO members reduce the number of US soldiers deployed.

There is no all else equal here. If your new member is the Democratic Republic of Congo the number would go up considerably. With Sweden/Finland it might very well go down since the border is far more defensible. But the US has a global footprint and those troops in Europe are nearer to potential conflicts in the middle east and northern Afrika, so it might make logistical sense to park them there anyway.