Explaining the question and answer a bit more clearly here. Question: How to create one purchase order for multiple material request records? Answer: Create a button on material request report and create a function for that button. Give an argument name (eg:records) and choose their type(material request)

po_id = records.ID;
   openUrl("#Form:Purchase_Orders?Request_ID=" + po_id,"same window");

In purchase order form, make sure to have a multiselect lookup field to material request form. Here Request_ID is a multiselect lookup field. Then, create a workflow for the load of the purchase order form to auto populate the details of the records selected.