Pages v7.3 does not support Mail or Data merge from Contacts. Although the Contacts application itself has label awareness in its print panel, it apparently only offers the selected address as a single label and not a full sheet of the same label.
If you are in the U.S., Avery has begun providing templates in Pages v7 format (yellow icon). So for instance, I download the Address Label Template 8160 from this page, click the Download link for it, and then sign into my Avery account — which triggers the download. These templates are in Layout, not word processing mode.
I open this 8160 label template in Pages v7.3 and if I know that I will always want the content of the 30 text boxes aligned in the upper left corner, it is better to do this now by pressing (⌘) while selecting all 30 text boxes. Then on the Format : Text panel, I set their alignment left, and their text box contents to the top:

Now, all 30 text boxes have this setting. From the File menu, Save As Template.
Now, open this template in a new document. In the first visual text box in the upper left corner, enter the address that you want repeated on the other 29 labels.
Copy and paste the following AppleScript into Dock : Launchpad : Other : Script Editor. Click the hammer icon. Then run. This script will replicate your initial address content to the other 29 text boxes — provided that the Pages document is open with this address template, and the first visual cell has your address entered to be duplicated. Before you save the finished list of labels, you may want to visit the View menu : Guides : Hide Guides. Now you can save a plain sheet of labels, and/or print them on your corresponding label sheets in the printer.
tell application "Pages"
tell front document
set labelLst to every text item of it
repeat with i from 1 to (count of labelLst)
if object text of item i of labelLst does not contain "Place Text Here" then
set itemPtr to i
exit repeat
end if
end repeat
repeat with i from 1 to (count of labelLst)
if not i is itemPtr then
set (text item i)'s object text to the (object text of text item itemPtr) as text
end if
end repeat
end tell
end tell
For the 30 labels in the Avery 8160 template, the script takes under 3 seconds to finish.
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