To check how many subscriptions you have used, or how many you have available, follow these steps:
Select Settings on the left-hand side of the Admin Portal
Select the Subscriptions tab
You will then see the following information
Total Available Subscriptions - the total amount of subscriptions assigned to your account.
Used Subscriptions - the amount of subscriptions you have used.
Available Subscriptions - the amount of subscriptions you have remaining.
Note: Making users inactive will free up subscriptions and allow you to add additional users.