sorry let me clarify, your merc camp can hold a number of armies. anywhere from 1 to 5.
when you place your merc flag in battle all the armies you hired at the camp will deploy. so if you had 3 different armies, all 3 will deploy eg 1 elephant, 6 archers, 6 spearmen.
once they have been deployed for that battle they are lost no matter what. so you can only use them for 1 battle much like donated troops.