Stocking unit must be your lowest unit of measure.  So you would set this up with a stocking unit of EACH (or UNIT, or whatever you want to call the things in the bag).  Then set up another unit called BAG, with a conversion factor of 50.  In your Vendor details or PO Contract Costs, you can set it up so that your default purchasing unit is BAG, and sage will understand that it contains 50 units.

Thank you!