While "mapas" is plural, it can also be used as an indefinite noun, meaning "some maps" or "any maps".
Here are some examples:
* "Necesito mapas" - "I need maps." (Indefinite, meaning "any maps").
* "Hay mapas en la mesa" - "There are maps on the table." (Indefinite, meaning "some maps").
The word "mapas" remains the same whether you are referring to a specific set of maps or simply any maps in general.