"Mercenary Slots
You can only have a single mercenary active at a time. If you wish to switch to a different mercenary, you will need to dismiss your mercenary and go to a liaison to hire another one. This step can be bypassed by buying additional mercenary slots with Station Cash. You can buy up to five additional mercenary slots. Also, the Mercenary Recruitment task grants an additional mercenary slot, it requires some crafting (300+ trivial) and a Triumvirate Blood Stone which is very rare drop off VoA nameds. Completing the Achievement: Master of Alaris will also grant an additional Mercenary Slot."
quoted from:
http://everquest.allakhazam.com/wiki/EQ:Mercenaries
(Evidently, mercs also take a group slot...)
I can't speak on the current state of mercs or what plans are in store for them..but, they would likely see changes before bots since they are a feature on live.