Instead of matching by customer names, you should match by a serial Id type customer number. What if you get two customers with the same name or a customer changes their name?
If you establish this relationship between your tables:
Table1::CustomerID = Table2::CustomerID
Then you can define this calculation field in Table1: Count ( Table2::CustomerID ) to compute a count of all the related records in table 2 for the matching record in table 1.
There are also ways to create a summary report that groups your products sold by customer and uses a summary "count of" field to report just the total product records. This approach does not require an added table, but also will not list customers for whom you did not supply any products at all.
If you want to try the summary report method, this link may help you get started: Creating Filemaker Pro summary reports--Tutorial