If your invoices have numbers, but they aren't showing when you print them, make sure you have the setting enabled to create transactions numbers.  For more information, see article Transaction numbers on sales forms.  

Next, make sure your numbers are selected to print on your forms.  Depending on your version of QuickBooks Online, there are two form customization menus. 

Custom Form Styles

  1. Select the Gear icon at the top, then Account and Settings (or Company Settings).
  2. Select Sales from the left menu.
  3. Select Customize look and feel.
  4. Select Edit next to the style you want to edit.  You can also create a new Form Style from this page.
  5. Select Header on the left.
  6. Check the box next to Form number.
  7. Select Save.

OR

Customize Sales Forms

  1. Select the Gear icon at the top, then Company Settings.
  2. Select Sales from the left menu.
  3. Select Customize look and feel.
  4. Select Edit next to the style you want to edit.  You can also create a new Form Style from this page.
  5. In the Header Information section, check the box for Form number.
  6. Select Save.

Now, when you print an invoice the numbers should print as well.

Also, the above steps can be accessed from within a sales transaction itself by selecting Customize on the bottom task bar.
 

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