A series of photographs can be considered a single work if you sell it as an inseparable whole. If you sell the photographs separately, then it is not a single work but as many works as there are photographs in your series.
If you consider your series as a single work, then upload an image of the entire series in your folder to show how each photograph should be arranged in relation to one another in an exhibition. And upload one or two other images of one or two individual photographs from the series to clearly illustrate the theme of your series. It is not necessary to submit images of each photograph in the series and in fact, it is not possible, as the application folder is limited to three images per work presented.