The empire is vast, so having 2 untouchables occur in it isn't particularly common! The only characters in the actual game which are untouchable are one of the assassin sub-types, which makes them pretty rare (and Human only?)
Narrative imperative to make good stories is kinda going to increase their occurrence in the novels. They are stated as being much rarer than psychics, and psychics are also rare on a population scale, but the Empire still finds thousands each day to send to Earth to feed the Golden Throne. This means that while untouchables are rare on a planetary level, there are probably going to be 1 / planet which still adds up to huge numbers across the entire Empire.