Thank you for posting in the Community. I'll be sure to point you in the right direction about showing the Total Quantity on your invoice.
In QuickBooks Online, you can customize the invoice template to add the quantity of each item on the invoice. However, it doesn't currently have the option to show the Total Quantity.
It's my priority to provide you with the best experience possible. I suggest letting our Product Development Team know that adding the Total Quantity is beneficial for your business. QuickBooks Online is constantly improving thanks to the ideas provided by users, like yourself.
To send feedback:
Go to Help at the top menu bar.
Click on Send Feedback Online.
Select Product Suggestion.
Enter your feedback or feature request.
Click on Send Feedback.
I'm always here to help and answer any questions that you may have. Wishing you and your business continued success.