Calf-high or knee-high wool or synthetic socks that sit above the top of the boot, which stops the rubber rubbing on longer walks. Four to six pairs covers a typical voyage depending on laundry and trip length. Bring a mix of medium and heavier weights so you can adjust, and keep a dry spare available for the mudroom.
The counterintuitive part is that more sock can make you colder. If the bulk makes the boot tight across the toes, instep or heel, you compress your foot, restrict circulation and get cold spots and blisters. One well-fitting wool sock is often warmer than two layers crammed in.
Sock layering is optional rather than a rule. A thin liner under a medium wool sock helps some people with friction and warmth. Bring both a lighter and a warmer option and settle it after your first landing.
If you have circulation concerns, follow your own medical guidance rather than a generic packing rule. And don't discover on the first landing that your thickest socks make the supplied boots too small.