Retainers are a very valuable asset in Final Fantasy XIV. From storage to gathering loot in ventures, these NPCs will follow you for most of your adventures in Eorzera.
You will not start with any retainers available, and you'll need to recruit the first two. Don't worry though, those are free and really easy to get.
You need to complete the quest called "The Scions of the Seventh Dawn" in order to recruit the first two retainers. Once that's done you may talk to a Retainer Vocate located in major cities (X: 8.3 Y;111.15 in Limsa Lominsa for example).
After your two first free retainers, the only way to get more is by paying for up to nine retainers in total.
FFXIV How to get more retainers
To get more than two retainers players will need to head to the Mog Station. Additional retainers are a paid option on your FFXIV account.
Each retainer comes to the price of $2 for a total of seven additional retainers.
These retainers can be used just like the free ones and will last for a month. If you decide to not continue paying for additional retainers for a given period of time and come back to it later, the paid retainers will come back in the same state you left them, without losing anything.
You can hire more retainers by visiting the official website here.