I was thinking (after misinterpreting and answering another question) about collisions on the atomic scale. I know it's impossible to visualize, but I did. For example the collision between an electron and a heavy atom. There are infinite ways the electron bounces of, dependent on the energies of both and their relative momenta.
But then I thought: How many atoms (assuming they can form a whole), packed in space(time), are needed to bounce off another package of atoms, to make the collision classical?
To put it in other words: chances are no longer involved, so the collision becomes non-quantum mechanical, i.e. classical. Within the framework of non-relativistic QM.
I guess this is the same as asking when quantum mechanics becomes classical (it's a blurry line, I guess again, which divides the two). And maybe somehow this has to do with quantum decoherence (see the picture below), but I'm a bit lost here. So..