Create an line number based on The Invoice Number

I have a master table Invoice with a fiels Invoice_Number and a detail table Lines.
Each Line belongs to a single Invoice.
In the table Lines I want to setup a field Line_Number and I want to automatically populate its value.
Line_Number must simply contain the line number of its Invoice.

For example : Invoice A has 3 lines numbered 1, 2, 3 and Invoice B has 2 lines numbered 1 and 2.

Does someone knows how to setup this field ?.
Thanks by advance.