Good day, AliG.

I'm here to help ensure your transaction number will show on the actual invoice via QuickBooks Online.

To show the number on the actual invoice, you need to ensure that the Form numbers option on your template is selected.

Here's how:

1. Click the Gear icon at the upper right corner and choose Custom form styles.
2. Locate the invoice template you're using and click Edit.
3. Go to the Content tab.
4. Click the upper part of the template or the Header.
5. Select the Form numbers.
6. Hit Done.

Once saved, you should be able to see the number on the actual invoice. 

In case you need help with utilizing your sales forms templates in QuickBooks Online as well as on what other information you can add, you can go through the resources below:

Customize invoices, estimates, and sales receipts in QuickBooks Online

Add, customize, or remove logos on sales forms

Please know that the Community is always here to help if you have any other concerns. Have a good one!