Even after someone clicks "unsubscribe" on one of your email blasts, Mailchimp will still count them in your total number of contacts list. However, you can easily remove all of your unsubscribed contacts so you aren't charged for them.
To do this, simply navigate to the Audience tab, then click "View Contacts" and sort by whether or not their subscribed. Then, select those unsubscribed contacts, and choose "Remove Contacts" to archive or delete them.