Step 1 - Create an integer variable to identify how many rows are in a repeating section. 

Step 2 - Add a number field to your repeating section and default the number to 0. 

Step 3 - Create a repeating section rule that updates the number field associated with that repeating section to the “count of rows” value with the integer variable you just created.

End Result

Note: The only issue with this concept, is if a row is deleted by the user, the numbering is off now. But hey, it’s something.