You could create a new table "VoucherCodes" with three fields:
- ID - A number field set to auto enter a Serial Number
- ClientIdFk - A number field that will contain the client's Unique Id number
- VoucherCode - A number (or Text) field that will contain your voucher code.
Every record will contain a value and so you will have 10 records with the same clientId but always a unique Voucher Code.
This does assume you have a Clients table with an Id field that contains a Unique Id per client (Ideally also a number field set to auto enter a Serial Number.
Then you relate the Client::ID field to the VoucherCodes::ClientIdFk field and you can put a portal on your Client layout to display all voucher codes related to this client.
In order to get the voucher codes into the VoucherCodes table you could do a simple import from a file.
And to relate the clients to 10 voucher codes you could write a script looping trough the clients and then assigning 10 codes to each one.
And please note that in the future, you might be able to use this field to auto-enter a unique voucher code each time you create a new record in this table instead of generating those codes externally.