Hi there, Learner97. I'd be more than happy to provide some insight into how QuickBooks Online works.
 

With an Advanced subscription, you can have up to 12 active custom fields for each type of form (invoices, estimates, purchase orders, expenses, bills, etc.) and profile (customers & vendors).
 

If you're interested in upgrading your plan, you can do so anytime.
 

Here's how:

1. Sign in as a primary or company admin.
2. Use the Gear (⚙️) icon.
3. Go to Account and settings.
4. Access your Billing & Subscription tab.
5. Confirm the payment info's up-to-date.
6. In your QuickBooks Online section, click Upgrade your plan.
7. Find our Advanced plan, then select Choose plan.
8. Follow the on-screen steps to finish upgrading.

I've additionally included a detailed resource about working with custom fields that may come in handy moving forward: Create & edit custom fields

I'll be here to help if there's any questions. Have a wonderful Friday!