How to put Serial numbers on invoice

How to put serial numbers on invoice automatically.?

That already happens. Manager finds the highest-numbered existing invoice and adds 1. This occurs as the invoice is created, not during data entry. (The reason for this is that multi-user installations might have several people creating invoices at once, resulting in duplication. Because the number is assigned only when Create is clicked, that problem is avoided.)

@Tut i am not speaking about invoice numbers, i need serial numbers on invoice items…
So kindly help me out with that if possible.

For now you can only use custom field and name it Serial No" until manager get new update (its on the to do list) that you can put custom columns this will be more more appropriate when entering serial number for items and showing on invoice.