Hi,
Our BoM template (excel xlt file) has columns headings with, for example, 'Column=Description' which brings in the headings from the library parts. Column 1 is used for the line item number within the BoM (which I think is what you are asking for, but if not it would work the same) = and in there we just have:
=ROW(A13)-ROW($A$12)
After our heading info, our BoM starts on row 13 and box A12 is left blank (so 0) and our standard template is pre filled for rows 13, 14 and 15. Altium then scales up from there as more line items are required.
If you do want a serial number of some form, then 'A12' (or your equivalent) could have the starting serial number required.
If the starting serial number comes from the design, A12 can be set to 'Field=Fred' where Fred is a variable set in the Project Parameters.
Hope that helps,