A good rule of thumb is to bring enough underwear and socks to change them every day even if you wear the same pants the entire trek.
The best option for socks is a combination of wool socks and lightweight sock liners made of a material like Capilene. Cotton socks will work as well, but you should make sure that you bring a fresh pair for every day. A good rule of thumb is to bring enough underwear and socks to change them every day even if you wear the same pants the entire trek.