I'll share with you the steps to disable invoice numbers in the program, @michellesaiz. This can be done by clicking the Customize button, and I'm here to guide you how.
Go to Invoices from the left menu.
Locate the invoice.
Click Edit from the Action column.
Select Customize at the upper right.
Turn off the Form numbers feature.
The screenshots below show you the steps.
You can always go back to the Invoices menu. From there, you can edit, duplicate, print, delete, and export the invoice as PDF from the Action column. For more details, see the Next steps: Manage your invoices section through this article: Invoices Menu.
I'll be right here to help if you need anything else. Have a good day, @michellesaiz.