Hey @Alberto , welcome to the forum! I’ve updated your post title in accordance with our question post guidelines: How to post in the Questions category
Make sure your title is a question you’d ask to a friend about Typst. 
I think they meant that the header should only be changed for the first (or a single) page.
By the way, please don’t use int(counter(...).display()), write counter(...).get().first() instead!