Seems like you also need a table of customers with one record to a customer.
SalesPersonnel::ID = Sales::ID_SalesMan
Customers::ID = Sales::ID_Customer
A calculation field defined in SalesPersonnel with this expression:
Count ( Customers::ID )
would render the count of customers served by each sales person without counting the same customer twice should they have more than one sales transaction with that customer.
SQL could be used, but you'd need to use the DISTINCT key word to filter out duplicates and it looks to me that you need a Join clause linking Sales personnel to their specific sales transactions.
Thank you so much, it works!!! really appreciated!!!