Invoice numbering is managed in the settings for Custom Form Templates in QuickBooks Online. Here is how to update this setting.
1. Navigate to Custom Form Style in QuickBooks Online.
Under the Gear icon in QuickBooks Online, open Custom Form Styles located under the Your Company header.
2. Edit the default invoice
From the list of invoices, select your firm's default invoice. You can either click on the row, or you can click on the blue "Edit" button on the far right.
3. Check the Form Numbers check box
Navigate to the Content tab in the center, then click on the top section of the example invoice shown on the right.
Check the Form numbers check box to generate invoice numbers.
Now your invoices will be assigned invoice numbers!