Могут ли быть в документе больше одного тега header, main, footer?
да
На самом деле, не все так радужно-определенно.
Тут https://developer.mozilla.org/en-US/docs/Web/HTML/... 
написано:
Note:  main must not be a descendent of an article, aside, footer, header, or nav element.
Authors must not include more than one main element in a document.
И это является цитатой из www.w3.org/TR/html5/grouping-content.html#the-main...
При этом на этой же странице есть ссылка на https://html.spec.whatwg.org/multipage/semantics.h... как на Live standard, где говорится, буквально:The main element is distinct from the section and article elements in that the main element does not contribute to the document outline.
There is no restriction as to the number of main elements in a document. Indeed, there are many cases where it would make sense to have multiple main elements. For example, a page with multiple article elements might need to indicate the dominant contents of each such element.