Do unsubscribed leads count against your lead count?
Correct me if I am wrong, but I think you are referring to your lead allowance within Marketo? It's my understanding that Marketo doesn't count unsubscribed leads as apart of the 'marketable database' in which they charge you for.
How are unsubscribed Leads counted?
Unsubscribed leads should still count toward your lead database number, as they are considered "known".
Jennifer DiMaria is correct. After a bit more research it turns out that unsubscribes, bounces, and duplicates all count to your known database.
Yep.
See this article for ideas on reducing the size of your database: http://developers.marketo.com/blog/how-to-clean-your-marketo-database/
Remember, marketo only checks your count during the renewal process, so if you archive unneeded records, this should keep your costs down. Besides, you don't really need non Customer invalid emails hanging around.