How can I export numbers file to text for mail on iPhone?

On the iPhone, with the document open, tap the circled three dots icon > Export > TSV (or CSV if you want commas instead of tabs).

Another option is to choose PDF and then attach the PDF to the email.

SG

You can simply go to File > Export To > CSV in Numbers. A CSV file is basically a plain text file, so it works just fine for email.

If you prefer a .txt extension, you can right-click the CSV file, choose Get Info, and change the extension to .txt.