Hi there odininc and welcome to the QuickBooks Community!
It's vital for business owners to conform to laws set by the government when it comes to invoicing. QuickBooks Online gives you the option to customize your form templates in order to both respect those norms and add field that best suit the needs of your business. I can help you add your business number to your invoices.
In order to edit your invoice template, navigate to the Gear Icon in the top-right corner of your screen, then click on Custom Form Styles. Select the Edit button next to the template you wish to modify. Go into the Content section and click in the top third of the previewed form on the right side of the screen. Make sure that the option for Business number is checked and that your number is entered in the text box nest to it. Click Done once you've completed these steps in order to save.