You speak english ?
Of course there are page headers! HIT "Print" to see the document parameters on the right. Make sure the box(es) are checked to enable Headers (and/or Footers). move the cursor to that area of the thumbnail, and you should see the box appear that contains the Header itself. Click in it and start typing! all the text settings (font, size, justification...) are there, too. All of this is available to see on Apple's Numbers Help website.
J'espère que cela t'aidera
Bonne continuation