I've been lead to believe that in the Warhammer 40,000 universe, untouchables - those who are psychic blanks, and act to cancel psychic powers - are very rare. However in two separate series the main character has a friend/companion/awkward love interest who is an untouchable.
I'm refering to:
Jurgan and Bequin, from Ciaphas Cain and Eisenhorn novels, respectively
So are they really that rare? Or is it just that they are a useful story aid?
The empire is vast, so having 2 untouchables occur in it doesn't make untouchables 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.
While very rare, there are still hundreds of thousands of Untouchables in the Warhammer 40K Universe. Carrying the genetically-engineered Pariah Gene, written into the Human genome, only one person on average, on any given planet will carry the gene. Imperial propoganda also ensures very few people know about the Untouchables thus increasing the "rarity" of the group.
However, there are hundreds of millions of worlds in the Imperium and trillions upon trillions of humans populating the galaxy, the density of the Pariah Gene varies from world to world. You can expect hiveworlds, which the Imperium estimates they have 32,000 such worlds, where super-dense populations number from 10 to 100 billion inhabitants, to harbor more than a few.
The numbers of Untouchables is sufficient the Imperium employs them in a variety of support fashions and even created entire armies out of them at one time. See: Sisters of Silence.
An Untouchable is non-psychic, for they bear the Pariah Gene, which severs the bearer's mind completely from the Warp, effectively making them both soulless and immune to all forms of psychic assault.
Their minds are also unreadable by any known means of telepathy. The bearers of the Pariah Gene possess the innate ability to disrupt psychic abilities -- they are "perceived" by psykers as a completely unnatural "hole" in the reality of the Immaterium. Their mere presence is thus intolerable to a psyker.
Even in humans and aliens categorised as Level Sigma and below in psychic ability, Blanks cause unbidden discomfort and hostility. Close proximity or intimate contact with a Blank can cause a psyker excruciating pain.
The Soulless are very rare in the galaxy since perhaps only one person on an entire world will be a carrier of the gene in every human generation. There are no known instances of Blanks existing amongst the nonhuman races of the galaxy. Warhammer 40K Wikia -> Psychic Blanks
With the added lore around the Sisters of Silence it has been shown that the pariah Gene is far more common then has been previously shown. In the Horus Heresy the sisters have enough Blanks to field at least 2 major armies (one in the webway to stop the incursion of Chaos after Magnus Folly, and the other travelling with the Space Wolves to purge the Thousand Sons legion). While also maintaining and running the thousands of Black Ships that are used to hunt for Psykers. This on top of any that are not part of the sisters of silence, either because they have not yet been found, or because they serve in some other way.
With the fall of the Emperor the order falls out of favour and so becomes disbanded however it can be believed that the number of blanks continues at about the same levels.
