Email Mail merged document

Mailing merged documents is a post-merge, manual action. It is not an automated feature as part of the Pages Mail merge process.

You would set up a template with the Mail merge items, and once the merge data (contacts or Numbers data) has populated the document, then you save the document, and visit the Share menu > Send A Copy > Mail > (Document format e.g. PDF) to attach the document to a new Mail compose window.

With Pages v12.1, Apple did not augment the Pages scripting dictionary with Mail merge features, so it cannot be automated via AppleScript.

See Populate and create customized documents in Pages on Mac - Apple Support

It took Apple 8.5 years to add a so-called Mail merge feature back into Pages. The term is a misnomer as no mailing is actually taking place, not even in the more elaborate Mail merge implementation in the retired Pages '09 v4.3 application. You could be waiting a long time for automatic Pages integration with Apple Mail. However, you can use the Provide Pages Feedback menu item to request Mail integration with the Pages product team. They do not participate in these public, user-supported communities.

Ideally, if you want mass emailing, you need something that integrates with Mail. MS Word has an email message option to its Mail merge so your merge content is done in a Mail message template, and then Word uses Outlook to transmit each merged message. This is discussed here.

If you search in the Mac App Store for Mail merge, there are several applications there that may or may not permit actual Mail transmission of Mail merge content.