Hi Hector,
If the number of products is not too much, then you can try svu123 svu123 solution. If not,
I don't know if an easier solution exists, but, you can try this.
Now you have a query where you have a manual input, multiple values variable for product say VAR1
Add a CKF say CKF1 to your query where you count the products. (Add a formula as 1, in aggregation select counter for detailed values)
Then create a condition where you add as CKF1 = VAR2. Create a variable in this condition with customer exit.
In the exit for VAR2 read the values of the VAR1 and count them. Assign VAR2 to the count of VAR1.
Now what we do is we have a variable telling us the number of products selected. When the query is run, a client should have at most this number of products. With the condition we assign, we guarantee that the number of the products selected in the variable screen should be the same as the number of the products the client actually sells.
Hope I make it clear and hope it gives an idea
Regards
Yasemin...