Tabular Form Serial Number Auto Generation Oracle Apex

1. Copy and Pest the javaScript at HTMLHader in the page.
<script>
function setNextSortOrder() {
  var highest=0;
  $(".sortorder").each(function(){
    if (this.value != "") {
      highest=Math.max(highest,parseFloat(this.value));
    }
  });
  $(".sortorder").last().val(highest+1);
}

</script>

2. Create a item and use default sql and select max value.
   and copy the item at "tabular form attributes" as PL/SQL Expression or Function Default ....

3.Copy and Pest sortorder at tabuler form item.
Element CSS Classes : sortorder


4. Finally copye pest the code on "Add Row" button -> URL Target
javascript:{apex.widget.tabular.addRow();setNextSortOrder();}