You need some wind baffle without being totally wind-proof. I sometimes wear a synthetic jersey over a merino T shirt. You can also use a sleeveless gillet windproof and one with a mesh back.

Windproof material varies in its breathability. For cold winter use you need totally windproof. For more temperate conditions or higher work rate you need more permeability. Membrane windproof are not very permeable. Pertex material is pretty windproof. My lightweight summer windproof is too permeable for winter use.
If your base layer is wet at the top of a major pass, CHANGE IT. Carry a spare or use a mid-layer against your skin for the descent. Wet merino is less uncomfortable than wet synthetics but it will still chill you on a major cold descent.
If you are into multi day endurance races, wool will prevent you stinking like a hobo.

I wouldn't take advice on how much to wear for a particular temp, everybody seems to run differently. I dress much more than other riders. You need a layering system which is flexible the use it. I had to force myself to stop and change clothing in the mountains.

There are some cheap and effective ways to prevent chilling. Stuff a newspaper down the front of your jersey is an old pro racer trick. Plastic bags, maps etc work as well.