In the same way you can use __INDEX to reference the current Loop iteration, you can use the __PARENT_INDEX variable to capture the current Repeating Section iteration.
Example
The Element 'Parent Index Variable' 'below is contained within a Loop in a Repeating Section, and has a Default value of Parent Index. The value of Parent Index is currently 3, as the user is on the third Repeating Section iteration. If the user was on the fourth Repeating Section iteration, the value of Parent Index would be 4.
Using Parent Index with Mappings and Elements Types
By default, the value of Parent Index is a number and is not compatible with Salesforce Fields with the Text data type or non-numeric Elements (e.g. Picklists and Text Elements). Therefore, if the value of Parent Index needs to be mapped back to a Salesforce Text Field, and/or if it is used in a formula to be compared with a non-numeric Element, you need to convert the numeric value of Parent Index into a string. To do this, use the TEXT function beforehand: TEXT(__PARENT_INDEX) .