If you would like to find out how many items/products customers have bought, you can see this at an individual customer level on the customer profile screen, as below:
If you would like to see how many (and which) customers have bought more than X items in a date range, you would use the orders report.
1. Choose the date range you want.
2. Navigate to the Orders by Customer screen.
3. Choose the "Columns Selected" and select the data you want (selecting all can generate too big a file to download).
4. Download the customer list into a CSV file, and order the file by number of items bought.
All data used is randomly generated and for illustrative purposes only.