It's the number of exceptions identified during the current mapping execution.
You mean the exception records (i.e. Bad records) identified during current execution of mapping?
If thats the case , why is condition NumExceptions = var_count > 0 passes as true even when there are no exception/bad records identified in current execution of the mapping.
Yes, the variable does not exist outside the current workflow execution.
What value is the variable reporting in the workflow log? KB 333791 has an excerpt of the log messages you will be looking for.