Can I add GST # to tax field in email template?

You would have to add via a child theme or plugin and filer into the email template. You can see the actions and filters you can use here: https://github.com/woocommerce/woocommerce/blob/master/templates/emails/email-order-details.php

so for example right after the order form: woocommerce_email_after_order_table

If you want it in the tax field you would need to filter in here: https://github.com/woocommerce/woocommerce/blob/f230452cafcdbcb177ba29a64b84ea1dee54cd97/includes/class-wc-order.php#L2018

Ben

Awesome, I’ll have a look. Thanks Ben!