They just spawn depending on the level of Catholicism in a region. If you play as the Papal States you can control them but you can't still control their spawn rate. With them, you can kill almost anyone if the inquisitors' piety are high...I use these cheats to boost their piety to the maximum
give_trait this GoodDenouncer 5
give_trait this Purifier
give_trait this DivineProtection
give_trait this TouchedByTheGods