You can select all the lists the campaign will be sent to.
Search : This input allow you to search a list, it's refreshing alone after you search something.
Show only selected lists : Show only the lists you have selected.
On each list, you can see the number of subscribers. If you require a confirmation, then only confirmed users will be counted.
Campaign will be sent to a total of : the number of users who will receive this email. Each user is only counted once, even if subscribed to several selected lists. You can see on the picture before that the number of recipients is lower than the sum of subscribers from selected lists (one is subscribed to both lists)