I have to mention GST number of My company in invoice

Hello, hrdcareerchoice.

I'll ensure the GST number shows up on your invoice. 

You can go to the Taxes menu, then enter your GST number from there. It will automatically appear on the invoice preview (though it won't show up on the creation screen). Here's how to do it: 

1. Go to the Taxes menu.
2. Click the Edit GST button.
3. Select the Edit settings option.
4. Enter your GST number in the GSTIN box.
5. Tweak other options, then click Save.

Here's a visual guide on what will happen: 

Need a GST-compliant invoice? Please check this article for a guide: Customise invoice for GST.

If you need to customise other forms, this article can guide you through the process: Customise invoices, estimates, and sales receipts in QuickBooks Online.

You can also browse other articles here. We have a number of help contents that help familiarise users with the program. 

I'll assist you if you have other questions for QuickBooks Online. Looking forward to your future replies!