After you have entered the name of the campaign, you need to select its recipients. To do this, at the second step of creating a campaign you need to select contact lists or segments from the corresponding drop-down lists. You can also select tags, and all contacts marked with this tag will be added to the recipient list. You can simultaneously select as many lists, segments, and tags as you want. Camping will be sent to unique total contacts in whatever list/segment/tag combination you have chosen.
Under the tag drop-down list, you can see the total number of contacts to whom the campaign will be sent.