How can I raise my army number in legion?

The base amount of troops in each formation depends on the level of your training grounds buildings (one for each legion). Levelling these up to your max level will help here. But you can greatly increase your troops count with the right heroes.

Heroes with the right skills unlocked and levelled up will be able to hold a lot more troops in their wave of the formation. Check the battle reports for the attacks you you should see that the enemy had possibly more than 1 hero with good skills.

For orange / purple heroes the first skill will boost the troop numbers and then the 6th skill will increase this even further.