An ID number for identifying the customer suggests that you also have a related table of customers with one record for each customer. You should use a layout based on that table to produce your list of the top 200 customers.
Define cOrderCount in the Customer table as Count ( Orders::Customer ID ).
Sort your customer recorsd by cOrderCount in descending order.
Use Go to Record to go to the 201st record
Use omit multiple records to omit that record and all the come after it from your found set.
You now have your list of the top 200 customers.
A summary field in Orders can compute the total saved for each customer.
There is no related table. The ID numbers (NPI- National Provider Indentifier) are assigned by the Department of Health and Human Services to physicians - the "customers" in my data.
Then you should create such a table. It's possible to use import records with a unique values | validation always validation on the customer ID field to populate such a related table with one record for each unique customer ID.
It's also possible to use the "re-order" option at the bottom of the Sort Record dialog to get sorted groups of records with sub totals for each group but with such a large number of records, I think that approach will be much slower as well as more complicated when it comes to getting the needed sub totals and omitting those records not part of the "top 200".