Mercenaries are part of your army that you can hire anytime at the Mercenary camp.
Mercenaries are divided by four categories: Common, Rare, Epic and Legendary. The higher Mercenary’s category and level are, the stronger they are. You can find out mercenary’s category and level by hovering mouse over an icon at the top right corner of mercenary image:
To hire Common mercenaries, you need to earn black marks in tournaments (LINK).
You can buy unlimited amount of Common mercenaries, but their price in black marks will grow with every consecutive purchase. You can level up your Common mercenaries by upgrading their abilities; you will also need black marks for it:
If you want to choose a particular mercenary accompany your heroes in battle, you can choose them in mercenary menu before the battle start. Alternatively, system will auto-pick first mercenary in your roster.
You can use as many mercenaries of a kind in a battle as you want, however, you will have until cooldown ends before summoning next mercenary. Every mercenary has a unique cooldown time that depends on their category and level.
You can buy mercenaries of any kind in the Mercenary kind, offers, and in the mercenary menu between the battles.