How do I show my GST number on invoices?

There's no dedicated GST or VAT number field yet. You can add it to your printed invoice with a custom template:

1. Go to Settings → Store Settings → Invoice Templates and click New template (or Edit on a custom one).
2. Give it a Template name and type your GSTIN as plain text in the template, for example next to your store address.
3. Click Save template, then Set active on it in the list.

Print invoice on an order uses whichever template is active. Use Preview to check the layout with sample data first.

The template can also pull in the store and order details, such as store name, address, tax name, tax percentage and tax amount. The PDF invoice attached to the order confirmation email uses a fixed standard layout, so a GSTIN added to a template won't appear there.