Kindly include provision to mention the Tax No ( Both for Issuer and customer) in the invoice.
There's no dedicated field for tax numbers on Wave's invoices, given that those details are not required by every tax administration.
You could put your client's tax number at the end of their respective name, on the Customers page, and add your own as a default footer or note on the Invoice Customization page. This way, you wouldn't have to have to enter those tax numbers by hand every time. I hope that helps!