If the term "witch" is defined in the faerie tale sense as a woman with supernatural powers, then pretty much every woman in the Darkhive fits this description... they are all capable of healing woulds at an exponentially greater rate than people elsewhere, they all have to potential to tap into deeper powers if they choose to open themselves to the unknown.
If the term "witch" is more narrowly defined as a person who deliberately studies the loopholes in reality to invoke effects that would not normally be under the control of mere mortals, then there would be far less individuals fitting that description, but still a sizable percentage (maybe 30% of the population, male and female, actively study ways to gain the power necessary to survive and possibly gain an edge over the people around them).
If the term "witch" is defined in the Catholic Inqusition sense as an individual (typically female) who makes some kind of pact with the infernal to gain power, then the numbers are far lower still. A small percentage of all survivors in the Darkhive have considered making a deal with the Hiveguard, with one of the darker Symbiote Surgeons, or possibly among the less mentally or emotionally stable, making a deal with the Shellbrood. Of these, less than half a percent have actually made such a deal and have gained a benefit from it. More often than not, they wander off into the labyrinth of the hive, following some ancient map or rumour, never to be seen again.
The dark skinned Khar-Tui (and those who follow their cultural ways) are perhaps the most likely to come out ahead when engaging in infernal pacts. As consummate scholars and keepers of both law ad lore, they are less likely to go wandering off into the forgotten parts of the Hive, but more likely to know the right way to word a contract, and the right warding effects to have in place, to get the best out of any deals that might be made. On the down side, the kinds of creatures who are capable of granting supernatural power are a selfish lot, and due to the reputation of the Khat-Tui, many of them are unlikely to deal with the race at all due to fear that they might not get a good bargain. More often a member of a less scholarly culture will seek out the wisdom of a Khar-Tui solicitor before heading off into the hive to find their supernatural destiny. The fact that not many come back is of little concern to the Khar-Tui, and is is said that canny members among that race have cut deals with infernal beings to send a steady stream of unprepared souls their way. But that's all just rumour and innuendo from racists isn't it.
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