How do I get more reserve units?

Update your barracks.

You have to wait in order to get RESERVE UNITS, which are troops that are kept at base. When an army needs training, you use these units in training. When you run out, you cannot train missing/switched army units. In V2.0, you have Immediate Troop Reserves, which immediately send in reserve troops.

You'll need to upgrade your barracks, once they get to certain levels you'll be able to recruit more units. Tap the barracks, upgrade then check the 'i' button to get the exact levels for your barracks that you need to get to for more reserves.