PPO is provided on a subscription model and the monthly invoicing is based on the number of subscriptions that are available to you, irrespective of whether all the available subscriptions have been utilised in the form of users added.
The number of subscriptions can be seen on the Subscription Management page, which is available to PPO Administrators who have the necessary rights. Hover over Administration and select Subscription Management.
This page shows the number of subscriptions currently available, as well as the number of active users on the system:
For more information on how to manage subscriptions, please read the following article.