All the backpacking stores tell me one pair is all you need, now
that socks are so well made (smart wool and all). This works fine for me if I am on a trail, and wearing running shoes.

If I am cross country with lots of sidehill neither running shoes
nor one pair of socks cut it. I wear a pair of thin socks OVER my
smart wools, inside of ankle high boots. Better blister prevention
and cushioning from rock bruises.
If I fold down the top of the socks into the boot tops I get
better ankle support.