If you're not seeing an invoice number in your documents for QuickBooks Online, it may be because you have custom transaction numbers turned on.
When this option is turned on, QuickBooks Online will not automatically assign an invoice number to the invoice.
If you've assigned a custom number, like ABC-1000, and then turned off the custom transaction number setting, then QuickBooks Online will continue the count. The next invoice that QuickBooks Online creates will be ABC-1001.
To change the settings, go to the Gear Icon > Account and Settings and Sales.
Here's how the invoice will look like with Customer transactions numbers turned on:
Here's the invoice with the setting off:
Notice how the Invoice no. field is gone.