Katrina only counts the first visit from your character and any secondary character from your town. Any subsequent visit after the first visit on the same day by the same character will not be counted. Visitation at another town also counts. After the 20th fortune that she tells you (or anyone from your town), she will stop you when you try to leave and will suggest building a public works project for her. If she haven't suggested the shop then most probably the game haven't registered your 20th visit to Katrina or you have not visited her enough and you can just continue visiting her and eventually she'll suggest the PWP. You will not miss her suggestion because she will stop you from leaving her tent and ask you to build a fortune shop for her in the main street. Hope that helps, you can read more on here: