From: South Carolina
OK Let's look at the Third Corps here (III). It shows what it can hold. In this case I> 0/14 M> 0/14. You may also see H (heavy ships) L (Light Ships) G (Guard factors) C (Cavalry factors) or A (Artillery factors). Each nation is different, some do not get units (only France and Russia get A and they have to buy them) some can hold more than others. What a corp is, can be thought of as a way of organizing your field army. So in this case we have the III Corps which is part of our mobile forces.
The I (infantry) and the M (militia) differ from the other types in that they count TOGETHER towards a total. So in this case the III Corps can only hold 14 units be they infantry or militia so you will have a combination like 10/4 or 7/7.
Every economic phase (every 3 motnhs) you are charged $1 for having a corp on the board, $5 if it is a fleet corp in a sea zone (as opposed to a port). Any unused counters remain in your pool until you need them, see the nice picture above. You can purchase as many counters as you have available in your pool for $1 each and they are good until the next econ phase. As has been pointed out, when the factors you purchased have arrived, you need to put them somewhere. G A C can only go in a corp. H L sit at whatever port you built them at until a ship counter comes and picks them up (or you can buy a counter and place it in the port, picking up the ships that way). I/M can be stored in your cities or on your depot markers. So if you have G A C coming up in the next 3 months and no room in the counters you have on the board, you are going to want to purchase a corp counter that phase to hold the new units that are arriving in the next 3 months.
You have a limited # of depot markers, in your case 8 (I think) which can be placed for free at the setup phase, cost $1 for every one that is out on the econ phase, and cost $1 to put on the board, although you can take them off the board for free. I always place all my depot markers on the board at the start (since they are free to pick up if I do not need them). Depots are placed every land phase, so each month.