Line number in subform

[JeffreyDavid]

I have a form with a subform and each time the form is opened, the subform shows 1 line and then adds lines acording to user input. I'd like to have each line numbered as a reference number.
Any ideas?

 

[dcx693]

I've never done this, but this link might help: How do I create running sums / totals / counts / averages in a Query?