Within Testersuite it is possible to activate users up to the maximum of your subscription. For example, if you have a subscription for 50 users you can activate up to 50 users within a month. If a user has been activated he/she will remain active for the entire month. So even if it is set to inactive again afterwards.
Users can be managed in the Administrator portal. The number of active users in the list of users is the number of users that are currently set to active. This is not the number of users used to see if you are staying within your subscription. The user report (at the top right of the user list) allows you to view the number of active users per month. So this is the number of users that are (have been) active in a month.
It is recommended to put users on inactive before the end of a month so that those freed up licenses can be used for other users starting the next month.