Dear Lal,
We are using PRT as a tool to inspect the material. So we are not concerned about which material we are testing, but number of tests.
Since this is not a standard process, we have to search for an enhancement or BAdI.
Material quantity shall be obtained from inspection lot, No. of inspections shall be obtained by no. of lots.
PRT shall be assigned in inspection plan / routing / Master recipe.
So you have to update this counter on every inspection (On usage of this PRT), this shall be achieved through ABAP development.
Additionally if you are interested, you may get PRT tracking also, please refer the link,
http://help.sap.com/saphelp_46c/helpdata/en/7c/93833855798671e10000009b38f842/content.htm
Regards,
VINOD.V