That’s now how data merge works. You set up ONE PAGE, add the data fields, and allow InDesign to create the new pages.
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You can't import more than one Data Source per document. So there's no way to create a doc with 16 different Sources.
So, I'm afraid you should consider creating one document per .txt source. The doc must have only one page. While running the data merging process, INDD will create a new document, with the relevant amount of pages.
You have to repeat this process for each of the 16 "template" documents.
If the final idea is to have a single document with all merged pages, you can consider using the book feature, and import the merged documents into it.
Thanks for your help,
now it works.