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

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