Google Data Studio dashboards rely on Google Accounts (G-Suite, Gmail). By default when you install a connector and link accounts, all accounts which are linked to the Google Account will be available in the dashboards.

As an agency you might want to separate different accounts into different dashboards.

There are two ways to do this:

1. User a separate Google Account for each client and connect the respective ad accounts only
2. Use Data Studio filters on a report level. This feature can be found when you head to File → Report Settings → Report Filter

If you use the second approach keep in mind that other users with Edit access to the report are able to remove the filter and see all accounts.