Hi,
Purchase order can be created for material with material number or with short text(without material number) or for with service master(activity number). Each item entered in PO line item represents as line item number 10, 20, 30, and 40 etc.......
For Materiel purchase order with material number , you can have Schedule Lines for the item(material) with quantities and delivery date for that material separately.
Regards,
Biju K