I'll help you find it, @sms2friends.

I've other customizations steps, so you'll quickly see the cumulative amount and the number of items sold in QuickBooks Desktop. 

You can run the Sales by Customer Detail report in your account. Then customize it to show the total amount and number of items sold on each of your customer invoices. 

Here's the guide: 

1. Follow the step 1 to 4 provided by KlentB above. 
2. In the Columns field, type Amount and Qty, then put a checkmark. 
3. Select Customer Type or Customer in the Total by drop-down. 
4. In the Sort by a drop-down arrow, choose Name. 
5. Go to Filters, then select the following: 

• Item - All sales items
• Customer Type- All customers/jobs or Multiple customer types
• Transaction Type - Invoice 

      6. Click OK. 

For more details about customizing reports, visit the article provided on this thread. 

In regards to the tutorial link on the Power BI app, I'd recommend contacting their Customer Care Team. They can provide you articles and videos on how to manage the application. Please visit this page to know their contact number: Power BI assistance.

I've included an article on how to memorize and combine reports from two or more company files: Understand reports in QuickBooks. 

If you happen to have other questions with QuickBooks, please post them in this thread. I'm always here for you. Have a lovely day.