To successfully field a large force you need a character adapted to it: high charisma for high leadership (reduced troop cost) and prisoner management (more captives = more money).
Alternatively instead of fielding a big army, field a better one. Companions are inexpensive and very powerful with a little nurturing. If you train them as cavalry and max out path-finding you can have an extremely nimble strike team able to chase down or run from any opposition on the map, letting you follow friendly lords and join their assaults or lead enemy lords into clever ambushes. Giving your companions blunt weapons is a great way to capture prisoners for ransom. Sea Raiders are excellent training for companions: they provide good experience, reasonable equipment, and low threat (companions can't die). Once you can successfully mow down Sea Raider bands at 3:1 odds or better, you're probably ready to take on the mercenary contract and fight in the big leagues.
If you have some (or all) companions learn trainer and diligently save your cash while they're leveling up, by the time you're ready for open war you can very quickly train and afford an expanded force of stock troops. If money gets tight again, just dismiss your army and hunt bounties with your companions until you can afford to recruit another sizable force and get back in the fray. Alternatively with the right gear and orders a high level and well armed band of just companions is often sufficient to defeat entire armies of lesser troops - but make sure you have a medic to patch them up between waves.