Are mercenaries already in the game?
If merceneries work the same as D:OS2 then it is essencially an ability to create more custom characters - as many Tavs as you want. At this moment you can do that by hosting multiplayer group by yourself, but there should be in-game option for that once the game is finished. In D:OS2 there was NPC that allow you to recruit (create) a mercenery after act1. In PoE1&2 you can recruit (and create) merceneries at any tavern.
I would make a distinction between creating multiple custom characters at the start of the game (Tavs with tadpoles and main-plot relevance) with hireable mercenaries (who will presumably NOT have a tadpole, won't have any dialogue options other than "join/leave my party," and might not even be able to talk with other NPCs.) I'd only classify the latter as "mercenaries."
BG3 will allegedly have mercenaries upon Full Release, and possibly the ability to create a fully custom Tav party in single-player mode.
