Different warriors occupy different number of places in your Army (e.g. one Knight occupies one place in your Army while an Orc occupies 2 and a Troll occupies 5).
You can find these values in the unit description at the Barracks.
Your army size depends on your Headquaters. By upgrading your Headquaters or building new ones you increase the army size.
How to command your Army:
The game mechanics doesn't allow you to command your warriors directly in the battle. But you can deploy your units according to their primary targets.
You can find the primary targets in units' description in the Forge.
No one returns from battle:
All troops that are alive in the end of the battle will disappear after the battle is over, and you will have to make new ones, so you have to consider how many of what troops to send into battle.