You don't need an add-on for that. Since Confluence 5.8 it is a standard feature and works as follows:
Go to the table and position your cursor where you would like to insert the numbering column. Then go to the menu on top and click on the cog icon. You will see "Table settings - insert numbering column". Just tick on that and the column will be inserted automatically.
No problem with erasing rows later on, the numbers will be adjusted automatically.