I occasionally have a pair priced as a set, but usually sell single mugs. I do like to photograph them in pairs though. When people want a set of 2 or 4 ....or whatever - there's nothing stopping them from buying multiples, but if you have them priced individually, they are more likely to sell. I find a lot of people who use handmade pottery like to have a selection from different artists, more like a collection that they use instead of a matchy-matchy set.