I need to write a footnote per page in my document (not a footer which is the same throughout document). How do I do that? Note that some page have no footnote(s)
A footer is defined in it's page style. If you want a different footer for each page, you also need to have a style for every page. See this help page for more information.
Note that you can also insert fields that vary based on, let's say, current page number or current chapter title. Creating a page style for each page seems a bit cumbersome.