Post-it is a brand name, like Kleenex or Xerox . They started out as adjectives: Post-it notes, Kleenex tissues, and Xerox photocopies. They then became a sort of eponym, and therefore became post-its, kleenexes and xeroxes, and eminently countable: one post-it, two post-its (which means two little sticky papers-- for pads, you'll have to specify 'pads of post-its').
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