Auto-numbering rows allows you to keep track of the quantity of rowed items as you add more in your default table.

Before setting up the formula, you'll need to ensure you're using a 'default table' by navigating the cursor to the left tab and clicking 'Default table' under 'Table'. (Using a pre-filled table wouldn't make sense as you can pre-fill the item number as a text field.)

The numberings are usually the first column of the table, however you can choose which column is your designated counter. Hover over to the cell and change the field into a 'Formula field'.

or

Tables cannot be set to automatically be numbered such as 1, 1.1, 1.2, 1.3.. etc.

As an alternative, you can opt to create different tables for the items, such as Table 1 is intended for '1' and Table 2 is intended for '2'.