Warhammer 40,000: Mechanicus

You start the game with 2 Tech Priests and are able to unlock four more by completing missions for Khepra. You should get them as a mission reward for her 1st, 4th, 7th, and 11th mission. You can check how far along you are on the View Campaign screen in Missions. If you're not getting any more missions from her then you just need to keep progressing through the campaign to unlock new areas.