I have legions, not really any method to how many I have, just where they go to fight.
General
1st cohort
9 cohorts
2 Roman skirmishers
2 Cavalry
3 reservist cohorts, or hoplites or celts, really depending on where the legion is fighting, or;
Spearmen, or;
Archers
(or a mix of the 3)
2 slots are for special task troops:
Defense legions, which I generally put near fortified chokepoints which I create to protect myself from aggressive factions, will have 2 units of scorpions, or one scorpion and one heavy ballistae
Assault legions, which I generally use to go into enemy territory and clear areas out to cover a siege, will have scorpions and repeating ballistae
Raiding legions, which I generally use just to punish factions I don't like, or to pull their forces away from somewhere I plan to attack, will have missile cavalry, and,
Peacekeeper legions, which will generally contain some local skirmishers or archers for defending a troubled settlement from possible attack while the legion sits in there to keep it under control
I have 3 other sorts of armies too.
Support legions:
Made up of romanised troops and generally carrying a couple of cohorts to be folded in to damaged cohorts of the legion it is following, as well as spare artillery and other troops suitable for converting the legion to Defense, Raiding or Assault legions, but also carrying heavy infantry for its own defense.
Allied peacekeeping groups:
Small armies made up of local tribes used for fighting other local tribes. I have two in Italy, one of Luciani and one of Samnite, backed up with Brutii spearmen and cretan archers, and both lead by a Roman general. For roleplay value I send each against spawning rebels made up of the opposite group. So if a bunch of Luciani show up I will send in the Samnites. I find some glee in watching tribes fighting against each other in Rome funded warfare with no Roman citizens having to die for it >.>
I think I get a feel what is was like for a real Roman commander sitting with a glass of wine watching two groups of barbarians killing each other so that good Romans don't have to dirty their hands with it xD
Client state free armies:
These are big forces made up with pure troops set to defend and expand around a client city. I have at the moment 2 of these, have had 6 in my current campaign; they are generally used to keep the people under control in their own area, but occasionally one raised to greatness. For example the Athenian army. When I took over Athens and installed Roman government I granted them client status and got to world building up the Athenian army. Athens was very calm, so I sent the athenian army for retraining in Rome, sent them back to Greece as one of the most elite forces in the world. If they had rebelled I wouldn't have been able to deal with them, and I didn't put a general or family member with the army for roleplay reasons, so it was a real possibility. I used the army to battle and eventually take control of the whole of Greece, kicking out the Spartans in a single glorious battle. They were eventually disbanded when Athens got Roman citizenship.
The Massalian army too was a good one, holding off about 20 attempted invasions from Gaul in the 60 years it was active :3