It's different to Rome in that there's no 'imperium' limit to how many armies you can recruit, but... one thing that caught me out is every army increases the support cost of all your armies. So if you've got two huge expensive stacks, recruiting two more lords, even if you don't recruit any units with them, increases the costs of the two big stacks as well. Quite a lot...

Also, confederating can drop your income drastically if your new confederates have a lot of armies.