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

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.

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