what i find useful is to disband all non-essential units. so if you have at least a region on all sides of your city, or it is hard to get to (like paris) you probably dont need anything but policeing forces. i just try to keep public order at a minimum of yellow.

pre-patch, i had like 4 full stacks in my marathas campaign, making like 25,000 a turn. now, i find it very costlly to maintain 2 armies in any campaign. i reccomend keeping 1 war going at a time, and not initiating war with anyone until that war is done. that way, you can justify having just one army.