Hi,
Standard SAP will suggest the GR quantity in MIRO along with the price, if GR based IV indicator is marked in PO and wsystem wont suggest the quantity and price in case this indicator is not set in PO. Now if you want to do multiple invoices for a single line item in PO, change the quantity and price manually in MIRO. You can do infinite times (depending on OMR6 configuration).
Hope its helpful.
Regards,
AKPT