You have five Familiar slots but you’ll only start off with one. As you get stronger and meet the level requirements, you’ll be able to unlock the additional slots. Don’t forget to deploy your best Familiars for their CP Boosts to count towards your total. Furthermore, the Passive Skills of all deployed familiars will be in effect.
You can set one Captain Familiar. The selected Captain will be the one that you’ll see accompanying your character. In addition to that, only the captain familiar will be able to activate its Active Skill so choose the familiar with an active skill that will complement your character’s strengths or address a weakness. For example, a familiar with healing capabilities is perfect for classes that don’t have access to reliable healing skills.