Being able to customize your invoice is an option which QB Online provides. I'll be happy to answer you question so you can back to doing what you enjoy.
QB Online has different invoice templates which you can choose from. So far, the Airy new template is the only one which offers the page number in the footer. In order to get rid of the page number, I recommend changing your template.
For more info, I recommend checking out this helpful article from our Community: Customize invoices, estimates, and sales receipts in QuickBooks Online.
Give this a try and if you need further assistance, don't hesitate to reach out to our support team using this link.