We have a special field called _row_number that you can use in the Calculated Fields.

Simply enter this value in a Calculated Field to include a column for the row number in your export:

{{ _row_number }}

Note that this will factor in the header row. So if you have a header row, then that would count as row #1.

If you want to start the count from zero, you can use this formula:

{{ _row_number }} - 1