The FAQ says hired units cannot be limited because you cannot count unrecorded/tracked deaths, but I think there may be a way around this.
While I am not a server owner or staff, I am fairly certain that the reason to track hired units of a player is to enforce server rules implemented for the purpose of fairness and lag reduction. If I am right, then simply identifying the number of units currently in use (IE in loaded chunks) by a player, should prevent most abuses of the system.
Players cannot use any of their units in unloaded chunks, and unloaded units also won't take more server resources. The only weak point is that a player could still potentially hoard troops so they can repopulate losses.
It works intheory, but I'm not sure how one could enforce it, because someone could hoard troops and then bring them all out to the same place, and then half of them despawn taking any valuable equipent with them. You'd have to figure out a teleport mechanic where it teleports your units to the closest unloaded spawpoint set by the player, or something like that. For servers, they could have a hidden plot where unused hired units teleport to, but there would be complicated (if not impossible) command blocks and there would be the challenge of making sure no enemy hired units massacre each other while in the "holding cell" plot.