Try an Advanced Query like this:
...then you can add columns from the product entity. So you can see what products customers matching the criteria ordered. You could start with invoice on the first level if you need columns of the invoice entity, but you cannot aggregate attributes in advanced queries, only by exporting the result to excel.
You could also add an attribute to the contact and/or account entity that holds the information for the amount of invoices the customer had and increment this number on every new invoice that is activated via a plugin.
As an alternative you can add your own rdlc report to the CRM: within the datasets sql query on the filtered views, you can aggregate, group and join.