milyenek is plural, and it is used here because az áruházak is plural and because it is not directly in front of the noun modifying it (as in milyen áruházak ...? "what kind(s) of department stores ...?").
Take it the following way - "az" cannot be an adjective. It always substitutes a noun, syntactically. Therefore it gets all the inflection the actual noun gets. And they are separated by a definite article.