In the
Code:
Data/world/maps/campaign/Imperial Campaign
folder you'll find a file called descr_mercenaries.txt. This file contains the data on the regions, what mercenaries can be recruited there, and how fast they appear. Here's a sample entry:
pool Syria
regions Assyria Syria Babylonia Coele_Syria Phoenicia Judaea
unit merc peltast, exp 0 cost 600 replenish 0.05 - 0.08 max 1 initial 0
unit merc eastern infantry, exp 0 cost 650 replenish 0.125 - 0.25 max 3 initial 1
unit merc arab cavalry, exp 0 cost 750 replenish 0.05 - 0.08 max 1 initial 0
unit merc bedouin, exp 0 cost 700 replenish 0.05 - 0.08 max 1 initial 1
unit merc bedouin archers, exp 0 cost 750 replenish 0.05 - 0.08 max 1 initial 0
unit merc elephants, exp 0 cost 4000 replenish 0.005 - 0.015 max 1 initial 0
I think the replentish is randomly determined from within that range, but I don't know if it's determined every turn, or just once. In any event, every turn the replentish number is added to the total. Once the total reaches 1, a unit becomes available to hire. Once hired, it's gone until another replentishment. You'll notice that the fastest elephants appear is .015 per turn, meaning at the very least it'll be 67 turns before the first merc elephant unit appears. No more than one can be available at a time, though it is region specific. You could hire one in Syria, and one in North Africa.