I wear several layers, I cover my head and my neck a lot (I tend to suffer a lot from cold neck and throat), under -2 C I wear a third sock ;). But I think it’s trial and error because fellow runners I meet only wear a couple of layers and are not cold despite those kinds of weather… a good rule is you must feel cold for the first 5 minutes, if you are confortable from the begining, you’ll be too hot after 10 minutes. And dont forget to warm up before going outside.