To see the records that are unsubscribed, please go to Email Marketing > Contacts and then click on the All Recipients button just below the green LIST button. On the grid on the left side, there are columns called Unsubscribed and Reason. If you filter the Unsubscribed column to true, that will show you all emails that are currently unsubscribed. To re-subscribe anyone, just check the box. You will want to pay attention to the Reason column before doing so to make sure that you do not subscribe anyone who had a hard bounce (unless you fixed the email address) or reported you as abuse.
Once you have marked everyone that you want to be subscribed, you can filter that same Unsubscribed column to false - that will tell you the total number of email addresses. You can check that total to the total on the list. If you need to add any, you can add them on the same All Recipients screen by selecting their name on the left side and checking the list name on the right side.