Additional licenses may be purcahsed directly in the Workspace of your subscription.
It is enough just to sign in to your account in Workspace, go to the Licenses section and click on the "Purchase additional license(s)" button.
Insert the amount of the licenses you would like to purchase
and select the payment method.
Once the payment method is selected, it is enough just to insert your payment data and finish the payment. After the successful payment, you will get the desired amount of unused licenses.