You need to write your own custom shortcode to display index in descending order. Please check the following related ticket where I've shared the solution to add the index in Ascending index using the incrementor shortcode:
=> https://toolset.com/forums/topic/display-item-number-in-views/#post-484259
You should adjust the shortcode to decrement the value.
This ticket is now closed. If you're a WPML client and need related help, please open a new support ticket.