It could be just one shirt. Although pragmatically speaking, I guess a shop probably will usually put all shirts of a particular model on sale, not just one particular shirt ;) And even if it is just that particular shirt, it isn't really important whether other similar shirts are on sale, too, so singular/plural is really sort of unnecessary information, at least if we assume that the situation is a customer inquiring about the price of a shirt.
-들 isn't very commonly used at all. If singular/plural can be inferred from context or if it's unimportant, you wouldn't usually add it. And for nouns which are not persons it is only very rarely used at all (so for example you will hardly ever see something like 책들). The environment where you will most commonly see it is when somebody is addressing a group of people: 아이들아, 이것을 먹으면 안 돼! (Kids, you mustn’t eat this!)
I learnt that -들 is only for living things so they may have applied that here but I'm not that sure.
In Korean can you ever say "this shirt?" I usually wear only one at a time snd often buy only one.