There are several ways to populate merge fields in a single document. On iPad or Mac, you can drag a Contact card over the desired merge fields. On iPhone, iPad and Mac, you can create multiple customised documents by merging information from a Contact group or Numbers spreadsheet, and preview how individual information populates before merging.
Go to the Pages app on your Mac.
Open a document with existing merge fields, then open the Contacts app.
Select the Contact card with the information you want to use to populate the merge fields, then drag the card to the desired fields on the page.
All fields of the same type (Sender or Recipient) within the document populate with the Contact card information.
Go to the Pages app on your Mac.
Open a document with existing merge fields, then click Document 
in the toolbar.
In a word-processing document, you may also need to click the Document tab to see Mail Merge.
Click Mail Merge, then click Merge.
Check the source of information for each field under Populate With, then click Preview.
Use the arrows to move through each record, then click Close when you’re finished.
Note: You can’t change information in the preview window. If you need to make any corrections, update the related information in Contacts or the source spreadsheet.
Go to the Pages app on your Mac.
Open a document with existing merge fields, click Document in the sidebar, click Merge, then click Merge on the bottom right.
A new document appears and contains each customised version of the document with the merge fields populated. After merging, documents only retain sender fields as merge fields.