Depends on the conditions. When it's really cold I carry two pair to hike (thick wool and thin synthetic under) in to keep warm, and one extra pair of sleep socks (alpaca). For general 3 season use, it's the same but lighter wool hiking socks because it's not as cold, or two pair of synthetic socks. Generally during 3 season I will wear both pair of hiking socks if it's cooler out, usually in the morning, and then switch them out--wash one pair and hang them on pack, wear the other one, etc. Summer/warm conditions I will only take two pair, a hiking pair and a sleeping pair, and one or both pair will be lightweight cotton.
Sometimes if it is just a quick overnight trip in warmer conditions I will just take one pair, though.