There's a few ways to do this, and it depends on if you want the value in Forms at all, or if you only want/need to have it at the workflow point.

If you only want it in Workflow, then you can just retrieve one of the columns variables from the table (not the entire table variable) and use a token function to get the value/row count.

If you want/need the value to be in Forms too, then you'd probably want to use Calculations and a hidden field instead.