We have a special field called _row_number (note that the first "_" is intentional) that you can use in a Custom Field or Calculated Field.

In a Custom Field, simply use the value:

{{ _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 a Calculated Field instead with this formula:

{{ _row_number }} - 1