Access Programming

In my access database I have a form with a subform.
This subform is used to enter multiple lines of data and ofcourse is all stored in various tables.

I want each line in my subform to show an item number, starting at 1 and increasing by 1 each line and the item number to be stored for later use in my tables.

When a new form is started, I need the item number in the subform to always start at one.

This is probably easy, and i have done it in the past, but am mentally blocked today.