In reply to waterbaby:
One pair, but the key is the fit. I have seen folks do all manner of things, two pairs, gimmick socks, changing boots to avoid blisters.
The straight answer is the sock should be good quality, have flat-stitched/overlocked toes, and be a snug fit, especially after washing, (which can mis-shape them), or when wet/damp in the boot. If you get any loose material, especially at the heel, or your feet don't match the heel position, they will be uncomfortable. The problem is that socks cover a range of sizes, with the consequent problems associated with that.
I have done 1000's of miles, the Munros etc, and once I realised a slightly snugger sock worked wonders, hey presto, ne'er a hotspot.