Each Hero has their own personalized token that you can earn in the Tower of Trials. Completing levels in this mode will award you tokens for your Heroes and help you upgrade their rarity, but also tokens for new Heroes. Once you receive a token for a new Hero, go to the Barracks to hire them.
 
The number of Heroes you can use at the same time is limited by your Barracks level. Upgrade this building and at some point you will unlock the opportunity to use more Heroes simultaniously.