I have found the solution for this thread. I used two variables with the next conditions and code:

Variable Name : TotalShipments
Val to Store :iif(tmpShipment=Invoice.Shipment,TotalShipments,TotalShipments+1)
Init value : 0
Reset at: End of Report
Calculate: Nothing

Variable Name : tmpShipment
Value to Store :Invoice.Shipment
Init value : 0
Reset at: Invoice.Shipments
Calculate: Nothing