How the number of subscribers is counted
Each plan has a limit to the number of subscribers on its lists. These subscribers are counted as the accumulated total of all the subscribers stored in all your lists.
If a contact exists in multiple lists then they will be counted multiple times, for example, an email address which appears in 6 separate lists will be counted as 6 contacts.
All your unsubscribes do not count towards the subscriber limit on your plan. Any hard bounces and complaints are automatically marked as unsubscribed and so, are also not counted towards the limit on your plan.