No, you do not need to have multiple Smart-ID accounts to use online banking services. Smart-ID enables to use all e-services that support Smart-ID. All you need to do to log in to your bank account with Smart-ID is to choose Smart-ID as the login method and fill in requested information (username and/or national ID number).
Smart-ID Basic account has more limited access, you can use it for online banking, but won’t be able to log in to other e-services.
Read more about how to upgrade your Smart-ID Basic account to full-access Smart-ID.