You can use the Sales by Customer Detail report to view the sales by quantity. Then, export it to Excel so you can sort for the top sales.
Here's how to get it:
In the Find report by name field, enter Sales by Customer Detail.
Click Rows/Columns. Then, mark the QTY box.
Click Run report.
Click the Export icon beside the Printer icon.
Choose Export to Excel.
Just click the Reply button if you have other questions.
Sales by customer didn't work, but you can go to the General Ledger account and add Qty and Products and service. Then, just export to Excel and delete everything but invoices. This will show you how much quantity in a given time.