How to update or add page numbers dinamically using javascript?

You can try using this, modify it to your needs if needed, it can be used in a button (like when you submit):

for (var i = 0; i < this.numPages; i++) {
 var pageNumber = "Page " + (i + 1) + " of " + this.numPages;//here edit the look of the number (e.g., 'Page 1 of 3', or  1/3...etc
    
 var textProperties = {
 cFont: "Helvetica",
 nFontSize: 10,
 nStart: 10,
 cColor: color.black};
    
 var position = {
 nHorizAlign: app.constants.align.right,
 nVertAlign: app.constants.align.bottom,
 nHorizOffset: -40,
 nVertOffset: 20};
    
 this.addWatermarkFromText({
  cText: pageNumber,
  nTextAlign: app.constants.align.center,
  nHorizAlign: app.constants.align.center,
  nVertAlign: app.constants.align.bottom,
  nHorizOffset: 0,
  nVertOffset: 20,
  nFontSize: 10,
  nRotation: 0,
  bOnTop: true,
  nStart: i,
  nEnd: i});}