Consecutive numbering when using templates

Hello together,

I'm currently creating a form in which following pages can be spawned by templates.
For this I need consecutive numbers on the newly created pages which are aligned with the last number of the previous page.
I also have a plan how I want to do this. But I need the prefix of the previous page. How to determine the prefix of the current page I have found out. But how do I determine the prefix of the previous page?

"

var prefix = "";
var re = /(.+\.)(.+)/g;
var res = re.exec(event.target.name);
if (res != null)
{
  prefix = res[1];
}

"

Thanks in advance for helping.