Have you actually logged onto the Mog Station and selected to buy the 7 retainer option? The game does not just give it to you without you actually going in and changing your previous 6 retainer plan to the 7 retainer plan. After that you just go to any retainer vocate in any of the main cities and you should be able to hire a new retainer. The 10th one comes from downloading the companion app onto your phone, and paying the premium plan for it. Once you have the premium plan set up, you don't need to do anything else with the app and can just forget it exists.