I am trying to set up the account. On our invoices, we require the VAT number of our clients but I can't seem to add them to the template invoices. Can you help ?

Welcome to the Community, @mauro-vanhemden-.

I've got the steps to show the VAT on your invoice template in QuickBooks Online (QBO).

To show the VAT on your invoice template, you need to make sure the Tax option is selected. Let me walk you through the process.

1. Click the Gear icon at the upper right corner and choose Custom form styles.
2. Find your invoice template and click on Edit.
3. Go to the Content tab.
4. Tap the middle part of the invoice template.
5. Select Tax.
6. Hit Done.
    

To help you utilize your invoice template in QuickBooks, you can go through this article: Customize invoices, estimates, and sales receipts in QuickBooks Online. This will give you more details on what other information you can add to your sales form as well as ways to personalize them.

Don't hesitate to let me know if you have other questions about VAT concerns. I'll be glad to help you out.