A series of photographs can be considered as a single artwork if they are sold in this way i.e. as a complete and indissociable series. If you sell the photographs separately then it is not a single artwork but is considered as being as many artworks as there are photographs in the series.
If you do consider the series as a single artwork then upload an image of the entire series to your file to show how each photograph should be arranged in an exhibition. Also download one or two images of a couple of individual photographs from the series to give a thematic overview of your series. It is not necessary to send (to the file) images of every single photograph in the series; besides, this is not technically possible as the application file is limited to three images per submitted artwork.