How many layers depends on your personal sweat factors and heat generating ability. What works for me is to use a cotton basic t shirt as the base layer, followed by fleece and then thermal jersey. Finish off with a coat that is either windproof or fluffy. If the weather profile will be varied add more layers thinly to make it removable. Ideally you want to make the setup just a little cold before leaving the house, then open vents or zippers before you reach the sweat point. Knee warmers help a lot under the tights, as can shoe covers and especially insulated insoles. For rides longer than about 30 mins, SPD cleat plates can suck a lot of heat straight out your feet. In extreme cases heat packs can contribute a considerable amount of heat to compensate, albeit for a limited amount of time.