counter for pairs of footwear
Used for counting pairs of items worn on the feet, such as socks, shoes, and tabi. Not used for single items.
I bought two pairs of new socks.
How much is one pair of these shoes?
The counter そく is derived from the on'yomi of the kanji 足, which originally means 'foot' or 'leg'. The use as a counter for pairs of footwear is a natural extension.