No, not always, but unless you know you are talking about one item only (e.g. one extra item of baggage - επιπλέον αποσκευή, (for one item) baggage size - το μέγεθος της αποσκευής), the plural is used. When talking about a specific item you also usually specify what type of luggage it is, e.g. βαλίτσα.