How do I increase the max number of troops in each tier of my garrisons?

Upgrading the barrack building will increase the max number of troops across all tiers of your formation. You can also place heros in each of the 3 waves once you have researched the corresponding items for the 3 spots. Many heroes have a skill that adds to the max troops number in their respective tier which you can unlock and level up to increase them individually.