Dear Friend,
First of all it is not very clear on what ground your principal to whom you are doing job work send the said invoices for taking credit? Is it a job work or Loan licence? It is a loan licence arrangment, then you can take cenvat credit.
With regard to your query as to how to take the credit of all such 15 Cenvat Invoices/Bills which are within the time limit of 1 year from the date of such invoices, you can take credit on each invoice in your Cenvat credit Account under independent entry in the said account.
Best Regards
Suryanarayana