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.
Here's how:
Click the Gear icon.
Under Your Company, click Custom Form Styles.
To create a new template, click the New style button in the upper right-hand corner.
Select Invoice.
Go to the Design tab to edit the template name, logo, color, font, and margins.
You can also go to the Content tab to customize the header, body, and footer of the invoice template.