Unfortunately you can't mothball armies, so if you're struggling with reinforcements, it's best to consolidate the regiments you don't need to reinforce right now.
In 1.23 though, if you get your army professionalism high enough you'll be able to send regiments straight back to the manpower pool. We'll have to see how this plays out in practice, but in principle it's an enormous boost to your regular army's flexibility and mobility: you can disband regiments in quiet places thousands of miles away, and then watch those troops immediately start feeding into frontline stacks (or you just rebuild the armies close to where they're needed). Could be pretty amazing for Russia for example, when shuffling forces back and forth between the Chinese front and the European front.