Not sure. Mine vary even amongst themselves. I think it largely has to do with what they encounter along the way. I've had companions show up for an army call with full ranks and then some (wounded warriors on the side which usually causes desertions later). I've had others show up with barely enough. On one occasion I had to rescue one of my companions who thought it was a good idea to attack an enemy who had superior numbers. They still have not gotten to lead another party yet. Their troops are also whatever they happen to pick up. Bandits, peasants, looters. They don't pick and choose.
Taking a quick look at Rhoe the Wanderer's party of 96 (clan rank 5), I see she may have been traveling a bit, that or managed to snag someone else's prisoners, considering I created her party around Odokh which is originally a Khuzait town in the East. Her party has Imperials, Vlandians, Aserai, Khuzait, Mercenaries, and looters. 41 different types/ranks of troops. I assume it took her a while to gather them, but I kind of let them do their own thing until I go on the warpath so I really don't know how long it took. Nor can I confirm if they level up or not. The only way I can think of to do so would be to give them a bunch of recruits and send them off to battle while sticking close with your own ranks full of recruits as well. Check after each battle to see if any of their troops evolved, and refill ranks to ensure they don't pick up more.
They also don't seem to be able to wholesale recruit towns/villages like the player can. They pick up 1 or 2, sometimes 3 recruits from a stop and they may do something else before recruiting more. They don't necessarily go looking to fill their ranks before engaging enemies. This increases recruiting time to max capacity as their current ranks dwindle from injuries/death.
