Create Quantity Field in Custom Field

[DariusV]

Joomla : 3.9
Virtuemart: 3.4.2

Hi All,
          I'm new to Joomla and Virtumart.  I need to track stock for each Size.  I would like to modify virtuemarts backend to look like attached jpg.   I have found the function where the custom fields are called from the database, but I can't figure out which page I need to modify for front end.
Any help will be much appreciated.

[GJC Web Design]

If u require stock management u should be using child products and multi variants etc