Thank you for joining in the thread, @saajid.
The VAT number should populate automatically on the invoices once you're done entering the registration number when setting up VAT.
Let's attempt using a private window and set up your account again. This will help us identify if this is a browser issue.
Depending on your browser, here's how:
Sign in again to your QBO account and follow these steps:
Once done, create a dummy invoice to check if the registration number shows up. If it works fine, you can go back to your regular browser and clear your cache.
However, if the issue persists, I'd suggest reaching out to our Online Support so they can investigate and check this further using their tools.
Should you need any additional assistance, I'm just a few clicks away. Cheering you to a great rest of the day.