Warhammer Fantasy isn't nearly as grimdark as Warhammer 40K, but the writers for Warhammer Fantasy still have a habit of putting very long lists of small grimdark things in almost every part of the setting at weirdly high density. Tome of Salvation lists multiple horrible fanatic sects associated with every major God, and one might wonder how these all fit in the same empire if they're "sects" of any notable size, rather than "five assholes with a banner".
Some speculative numbers pulled out of my ass, for a crude sense of scale:
The Empire has about 20M people.
90% of them will praise Sigmar on Sigmartag, beseech Rhya for good harvests, see their relatives interred in a Garden of Morr, pray to Ranald for luck when doing something risky, pray to Shallya for healing after the risky thing resulted in an injury, and more or less stop there. This leaves 10% (2M) with more than trivial devotion to some particular Cult.
90% of those will go on a one-off pilgrimage, make larger donations than usual, and/or find mundane work in a Cult which still needs lots of mundane work done like making clothes and building temples. This leaves 1% (200K) who might become priests or join a Knightly Order or Sect or the like for the long term.
90% of those will join a respectable mainstream order. This leaves 0.1% (20K) to be fanatics.
90% of those will be in the "eh, kinda weird but definitely pious" group one respectfully appreciates from a distance, like the fanatic Mannannites who swear to live their life on boats and never set foot on dry land. This leaves 0.01% (2K) to be grimdark fanatics.
The Empire is divided between about 10 major gods, so this leaves 0.001% (200 people) to join grimdark fanatic Manannite sects in particular, split among the boat-burning Manannites, the monotheist Manannites, the Great Flood Manannites, and so on.
Less than 100 people in each grimdark fanatic sect, and 2000 of them across the Empire in total, sounds vaguely plausible for something the Empire can treat as a painful footnote, not a crisis demanding attention.
tbh.
