You will never have a perfect battle in A Total War Saga: Troy, as you can expect to always take casualties. Your units may not die completely, but they will lose troops indicated by their hit points. You can see a unit's hit points by clicking on an army and viewing the score above its cards. Rather than recruiting a new unit, your troops are given additional army replenishment hours.
What is army replenishment?
Replenishing the army restores the health points of your unit after a battle. You can see which units are replenishing their numbers by the health icon next to their map while viewing your army information at the bottom of your UI. However, if you attack or walk around enemy territory, they cannot replenish their numbers. This only happens in territory owned by your faction. If you spread your armies too far into enemy territory, you may find that your army will not last long after several battles.
How to rebuild your armies outside your territory
The only way to rebuild the army in enemy territory is to change the marching position of your general in encampment. You can only do this if they have at least 50% of their campaign movement available. While encamped, your troops regain their numbers, but you cannot move the army until you move from encampment to another position, such as March or Raid.
The encampment position is a good way to stay inside an enemy's territory to keep your troops in good shape. It's also a good way to lure the enemy to a location if they think you're licking your wounds. The encampment position gives your units better defense in the event of an attack, making it a good way to take out an otherwise stronger enemy as well.
