Maximum at the moment is 8 retainers, yes. So you can rent from mog station 6 retainers. Also to note. If you for some reason cancel your retainer subscription, the retainer just becomes inaccessible in the game. They still retain their inventories which can be accessed again when you resub for correct amount of retainers. That is to say, no items in their inventory is lost even if there is a break in renting them.