Yes, don't separate the data into separate tables for each client. I know that seems simpler and and an easy way to keep all the data securely separate, but there are ways to accomplish that with combined tables and it will make your report much easier. If you insist on spearte tables, you have keep adding tables and layouts for each new client and you'll have to merge all the records into a combined table for reporting purposes anyway.
You can set up your tables so that each record is tagged with a customer ID. You can then use Manage Security to limit each client's access to only their records. Fully implemented, it will be difficult for them to even know that the other clients exist--though using FileMaker Advanced to set up some custom menus to better control some of the options in the Record menu would be a good thing to do here.
To get you started, look up "Editing record access privileges" in FileMaker help and pay close attention to the sub section titled: "Entering a formula for limiting access on a record-by-record basis".