Indexed numbered column

I do see similar posts to what I am looking for, but it seems like this would be simple DAX.  I have a class registration system for internal training here in my organization and now I am creating a Power BI report to show those who are registered for a particular day and session.  I would like the registrants to be numbered, in numeric order. (for those familiar, it would be a dim i as int; print i; i+=1).  The underlying datatable does have an id column, but this would not work as it is just an identity spec.  

I have found some examples of DAX that use RANKX, but maybe I am using it wrong.  I just need this numbered column (1, 2, 3, etc) for a filtered Class session.

Thanks