What happens to my mailing list when someone revokes his consent?

In short, nothing! The recipient's address remains as part of your list and the number of subscribers doesn't change. However, the address that revoked its consent is now registered in the Consents revoked section of the Consent list page, and in the VSL which tells the software to completely ignore that address in the future, so none of your campaign are ever sent to that address. Even if you add that address to a new list later, the software will remember to ignore it. The only way to unlock that address is to have your subscriber fill a new sign-up form and follow the double opt-in process.