I'm here to guide you through the process of getting the Invoice Number to print on the Invoice.
Let's begin with creating a template:
Click on the Gear icon, then Custom Form Styles.
Press on the New Style and select Invoice.
Navigate to the Content tab and hit the Pencil icon on the top section of the form.
In the Form section, mark the checkbox for Form Numbers to show the field on the form.
Once this template is complete, we can apply it to your Invoices. Go to the Invoice that you want to print, and in the footer, click on Customize then select the template you just created. Press on Print or Preview to look at your masterpiece. Here's a Community Article that provides more information on Custom Forms in QuickBooks Online.
Let me know how this goes for you! I'm just around the corner if you need further assistance.