I've subscribed to SmartEmail 10 but I can only create 9 email account, 1 is taken up by the administrator.

We have to create an domain administrator account (also a mailbox) on the server  to allow you to create additional users. Administrator can be reassigned, simple write to support@vastspace.net and let us know the appointed primary administrator whom is one of the 9 accounts you have created.
Once the new domain administrator has been assigned, you will be able to remove the unwanted mailbox to create your 10th mailbox.
It goes the same for SmartEmail 25, 50 and 100.