Choose clothing layers by temperature and activity.
Clothing insulation is measured in clo units, where roughly 0.8 clo is one typical garment layer. What matters is not air temperature but effective temperature: wind chill lowers it, and your own metabolic output raises it — hard running is worth well over ten degrees. Wet conditions cost another few degrees because damp insulation stops working, which is why a waterproof shell counts for more than an extra jumper.
Layering from clo requirement
Effective temperature = wind chill + metabolic offset - 3 C if wet; clo required = (33 - effective temperature) / 5.55; layers = ceil(clo / 0.8)
It depends on effort as much as temperature. Standing still at 5 C in wind may need three layers, while running in the same conditions needs one or two.
A unit of clothing insulation. One clo is roughly a business suit and keeps a resting person comfortable at 21 C; each ordinary garment layer contributes around 0.8 clo.
Because metabolic heat rises within minutes of starting to move. Dressing to feel comfortable while stationary almost guarantees overheating and sweat-soaked layers later.