How to put serial number column in Invoice & others printed docuemnst

Dear All,
wish you all Happy New Year 2021.
I like to ask, how to put Serial Number Column in the Invoice & Others.

Do you mean like a code for line items?

If you are asking how to number the line items on forms, see Add line numbers to themes | Manager.

If you are asking how to insert a serial number for an inventory item, use a line-item custom field. See Add custom fields for individual line items | Manager.

Dear Tut
Thank you for the answer, YES, my question is auto add line number, in the invoice , so that we know how many items, are in the invoice.
I got your answer, but writing or inserting code is beyond my IT knowledge level.
I will try to do it, but if there is a simple function like adding custom field function will be easier for me.
Thanks & Brgds

You already have the answer.