QuickBooks Online (QBO) doesn't have a dedicated field for project numbers. I recommend adding a number after or before entering your Project Name. You may also add a number in the Notes field.
Let me guide you on how to add a number when making your project.
Here's how:
Go to Projects.
Select New project.
Enter your project name and preferred number in the Project name field.
From the Customer dropdown, choose a customer.
Enter your preferred date for the Start Date and End date.
Select a Project Status.
You can add the preferred number of your project in the Notes field.
Once done, hit Save.
For your reference in managing projects, check out these articles: