I am from the two-pair (thin silk or poly liner and thick wool cushion outer) school of thought. However, and this is critical, the time of year, and your individual foot dynamics must be considered. Just as with hiking sandals, shoes, and boots, everyone has different feet. I doubt that one, standard solution would work for everyone.
For example, I walked the
Camino Frances twice (2013 & 2014) wearing the same, well-broken in boots (had them resoled - twice) and socks, both years. BECAUSE the weather is cool and wet in April and May, I appreciated the warmth and relative dryness the double sock layer provided. But, that was then...
In April this year (2015), I walked the 10-day Porto to Santiago route in much milder temperatures. During my first two Caminos, the morning temperatures ranged from 1 or 2 degrees Celsius to afternoon temperatures in the Celsius "teens." In April this year, the temperatures started in the mid-teens in the morning and went to about 27 (80F) in the afternoon. I experienced no rain until the final 36-hours into Santiago (in Galicia of course).
For these conditions, generally warmer and drier than usual for me, the double sock layers caused my feet to sweat excessively. They also swelled more than usual. I could handle the swelling, as my boots are oversized for it. However, I actually developed blisters on the balls of both feet, for the first time in three Caminos! Go figure!
I examined my feet and deduced that the perspiration in the boots caused it. The blisters were dealt with. I also started changing my socks for a dry set after several hours walking. But I was surprised.
My point is that I am training now with, and will plan to wear, a single, compound layer, thick(ish) woven poly sock the next time I am walking a Camino in warmer weather. I plan to test them when I work this July and August as a volunteer at the Pilgrim Office in Santiago. I will be on my feet all day and walking around the entire city daily. So, it should be a reasonable test.
The problem I encountered in April is that, at the last minute the weather forecast was for cooler and wetter weather than I actually encountered. My boots are too large to wear just the poly liner socks. They were sized to accommodate two pair of socks. The Smart Wool outer socks make my sweaty feet itch when worn alone. In any event, having the ankle-high uppers would cause excessive sweating regardless of the type of socks I wore.
I have plenty of these single layer, compound weave socks. I buy them at REI. They have lateral compression panels woven into ball of the foot and insole areas, so they provide very good lateral support. They are also available as low-ankle socks to be worn with sandals or low-shoes, or crew height that are good with mid-height boots.
I usually wear them as "knock-around" casual socks during the winter months. So, on returning home, I recently invested in a pair of Keen Arroyo II hiking sandals, sized to handle the same two-sock system as I wear in my April-May Caminos. However, and this is important, I also tried them with the single, compound woven socks, adjusting them and walking around the store so I was comfortable that I could adjust the speed-lacing system to fit the overall thinner socks. As a bonus, my custom-made orthotics fit both my mid-height boots and sandals exactly.
So, the plan for a warmer weather Camino is likely to be hiking sandals with a single compound layer hiking sock. However, to ensure that my feet remain fairly dry, I will bring an extra pair of dry socks to change into during the day. This increases my sock "carry" by one pair.
I either accept that weight penalty or carry the weight of additional foot fixing stuff. Based on experience, I think the few ounces / grams of prevention "outweigh" the several ounces / grams of palliative care supplies. We shall see.
BTW - for those of you who are brand and model junkies, here is some useful information relating to the Keen boots and sandals I wear:
3- Cool Season (late fall, winter, early spring) - Keen Targhee II mid height "waterproof" boots. The manufacturer indicates this boot runs ONE-HALF SIZE
SMALL. So, my street size 11 / 44.5 feet, upscaled for: two pair of socks (+ .5 size); daily heat-related swelling (+.5 size); and splaying after several weeks walking with a pack, require a size 12.5 / 46.5 boot. As these boots run 1/2 size small, the stock size 13 / 47 boot works PERFECTLY.
3- Warm Season (late spring, summer, early fall) - Keen Arroyo II hiking sandals. The manufacturer indicates this boot runs ONE-HALF SIZE
LARGE. So, using some of the criteria above, and factoring in the heat and added heat related swelling, I arrived at the same size 12.5 / 46.5 conclusion. But, considering Keen's size statement, the stock size 12 / 46 Arroyo II hiking sandal worked out PERFECTLY. The net fit is exactly like the cool season Targhee II boots. In fact, they use the identical sole / outer and tow box, so my feet are very well protected when dealing with rocks, gravel, and on downhills.
BTW - Another interesting feature of the sandals is that the manufacturer claims they are MACHINE WASHABLE. I regularly remove and hand wash the insoles, factory or custom orthotic every night after hiking anyway. But being able to toss the sandals into a gently cold water cycle after slogging through mud is a nice to have feature.
FYI - I have no ownership or commercial interest in Keen (
www.keenfootwear.com). However, the lasts (foot molds) they use to manufacture the shoes / boots / sandals are the closest I found to my feet.
The only other manufacturer that comes close is Mephisto, the French shoe company. However, they do not offer the right sort of boots and sandals for walking the Camino.
That is one reason why many serious walkers or people who spend all day on their feet frequently find one footwear manufacturer that suits their shape of foot, then remain loyal to that brand.
I hope this helps.