To get more units you must first construct the corresponding building. When a building is finished your unit cap for the specific unit is raised and you may purchase new units from the in-game shop.
Player wants to have more Armored Personnel Carriers (APCs). First player needs to purchase and construct a Motor Pool, which will unlock APC in the in-game shop while also raising player?s unit cap for armored vehicles by two from initial two to four. After this, in order to purchase more APCs than four, player will have to build additional Motor Pools.