I need to fill up my comp with hundreds of small falling objects so that the screen slowly fills up like a container. Im thinking about doing this in Newton but what I want to know is do I actually need to have hundreds of layers? One for each object or is there another way to do this?
Newton could do this - sort of - Each coin would need to be a different layer so you would first have to try a few numbers of layers to see how it filled the vessel. The problem is that newton doesn't know what the third dimension is doing, so the coins would all fall flat (or on the "fake" 3D angle your art gave them) and I don't think that would look very realistic because they wouldn't tumble. I also don't think Netwon would do a z axis rotation very well in this case - I think it would only know about bounces but wouldn't take into account any rotation that should be imparted even if it is only on the z axis, I could be wrong on this.
Add to that the tendency all 3D sims have when you pour a set of particles into a vessel - they tend to shoot up and out at a much higher rate of speed than they should - kind of a sling shot effect as they round the bottom of the container and pick up speed through angular momentum. I really don't know what it is about the math that causes this. You really have to ramp up your friction (and usually animate it) to overcome this and spend hours scratching your head on settings). Even mind bendingly complex sims like Next Limit's Real Flow do this. And one of newtons examples on their site shows this exact problem (or effect, if that's what you are looking for.)
You are going to run into this lack of 3D issue with every particle emitter that I know of for AE because AE is a 2.5D package, simulating the 3D environment. If you want real looking coins that can jam a penny jar then you need a 3D package that has collision detection for their particle system.
Stardust and Particular are the two biggies in particle emission for AE and both are 3D simulators and don't know anything about a particle's 3D volume so again no tumble. The coins would fall like they are trapped between two sheets of glass, one coin width thick.
I tried thinking of a way for Element 3D to team up with one of the above - I think its doable with a boat load of scripts but I would take longer to get it to work and set up than it would to call a friend with C4D who could literally do it in an hour (plus rendering).
Finally, coins are all about reflections, and you won't get any changing reflections because again they aren't tumbling so the same face is always facing camera. And one step further, coins reflecting coins would be right out.
Doing this in a 3D package like Cinema 3D is pretty simple (3DSmax too, Maya can do it but it's a deeper more complex engine to master, so simple for the software but harder for the operator) Some 3D compositors might be able to also like Nuke or Shake (with third party plug ins). I can't speak for Blender though.