How to Change Indexing Based on Change in Given Column

I have a dynamic table, in which data will aggregate (append) over time, such that the listed account numbers will grow, with possibly new account numbers being added to the list.

I need to have an index column, that starts with a 1, that resets the count each time there is a change in the account number column.

My table will sort by date, then by account number (lowest to highest), and finally by amount (highest to lowest) before adding the index.  

How would I add this dynamic index column after completing the above sorting?

The index number becomes critical at a later date, in the referencing and reconciling of the data.

Thank you in advise for your help in this.