First, you need table Occurences setup in Manage | Database | Relationships that look like this:
(You also have a link from ProdCust to Products, but that isn't a relevant relationship here.)
That gives you a relationship between ProdCust and ProductDetails that looks like this:
ProdCust::ProductID = ProductDetails::ProductID
Then you can put Count ( ProductDetails::ProductID ) in ProdCust to report the number of related detail records.
Note: I originally thought you needed: Customers----<ProdCust>---Products---<ProductDetails, but then I noticed that these details were specific to a given product and customer.