Hi Sandeep,
BAPIs are defined, external APIs. For example, there exist BAPIs to create a business partner.
When you set a value in the a Web UI the handling is performed by a combination of BOL, GenIL and existing function modules. I'm not aware of any BOL object that is implemented using a BAPI. So the answer to your question is 0.
What are you really trying to trace / trying to find out? Setting the relevant break points in the BOL and/or GenIL might be the better option.
Maybe this excellent blog by Sigrid Wieshofer will be helpful: Tips and Tricks - Troubleshooting CRM (Interaction Center)
Christian