Players can hire up to two (2) retainers per character, free of charge.

However, if you wish to hire additional retainers (a total of three or more), you can do so by subscribing to the Retainer Service on the Mog Station at https://www.mogstation.com.
 
For details about pricing, available payment methods, and other Retainer Service inquiries, please review our Retainer Service FAQ at the following link: http://support.na.square-enix.com/faqarticle.php?id=5382&la=1&kid=77420