So, this may be a commonly known fact, but I didn’t know about it and learned the hard way:
If you have extra retainers hired on your regularly recurring subscription, then decide to use a game time card (for whatever reason: someone gifted you one, in-game perks for use, whatever), the game time card will only allow you the 2 non-extra retainers during the subscription period and your extra retainers will not be available (they’re grayed out) until after the game time expires. At that point, you will need to go back into the Mog Station and “re-subscribe” to your extra retainers.
It was difficult losing the extra inventory space and loot stored in those extra retainers! I just thought to share so that others can avoid the pain.