How to add an business account number on a invoice to receive payment?

You've come to the right place, @Ravi3. I'll provide the two options to help resolve your concern. 

If your custom invoice template isn't Airy new, let's go to the Footer section under the Content tab. You can add your business account number from there, so you'll get paid. I'll guide you how. 

1. Go to the Gear (⚙) icon at the upper right and select Custom Form Styles. 
2. Locate your custom invoice template and click Edit from the Action column. 
3. Choose the Content tab. 
4. Click the Footer or last section of the invoice. 
5. Enter your business account number either in the Message to customer or Footer text box.
6. Select Done.

The screenshot below shows you the last four steps. To learn more about the process above, check out this article: Customise Sales Forms in QuickBooks Online (QBO).

Here's what it looks like after performing the customization steps above: 

If your custom invoice template is Airy new, let's utilize the "Your payment details" text box to enter the business account number. Just go back to the Footer section under the Content tab (see the screenshot below). 

Once you receive the invoice payment, go to the + New menu to record it. Doing so will help ensure your customer balances are up to date in QBO. 

I'm here anytime you have other concerns. Enjoy the rest of your day, @Ravi3.