I want to insert VAT number on the face of invoice- how can I do it?

I'm happy to see you here in the Community, @sureshmandalia.

You can add a custom field on your invoice template to insert the VAT Number section.

Here's how:

1. Click on the Gear icon at the top.
2. Press on Custom Form Styles under Your Company.
3. Hit on New Style and choose Invoice.
4. Refer to the Content section and tap on the upper portion of the sample template.
5. Scroll-down and tick on +Custom Field.
6. Put a check-mark on the the box and enter the name of the Field.
7. Tick on Done.

Once done, your invoice will now look like this:

To learn more about customising sales templates in QuickBooks Online, please refer to this link: Customise your invoices, estimates, and sales receipts in QuickBooks Online

You've got me here to help if there's anything else you need. Just let me know by leaving a reply below. Have a pleasant day!