Ancalagon
Dusty Dragon
I think we are disagreeing on what we consider rare. 1 out of 100,000 doesn't match how the setting describes spellcasters in full on magic schools.
But even so, There are TONS of mages all over the planet. Kislev is run by a witch and the witch control everything behind the scenes. Each elven nation has dozens of wizards. Lizardmen are run by super magic frogs.
The thing is WH RPG is mostly Tier 1. You tend to die before you meet any friendly casters. But if youreached the equivalent of mid Tier 2, you would know at least1 spellcaster.
Well that's my point. The average adventurer dies before they become important enough to ask a wizard for a magic favor.
But the wizards matter. Warhammer is a grimdark war setting afterall. The mages are walking talking war machines. They matter a lot.
But how rare are they compared to FR? Quite rare I think. You can't step outside without tripping on a wizard...