I was wondering if I could get a detailed explanation of corrective shapekeys and how it would work with different faces/poses in one animation

I’ve been hearing a lot about corrective shapekeys and while they look really useful for fine-tuning any poses with some weird things happening to them, I’ve only seen guides use them for just one pose with just one shapekey. I was curious how this worked for animations. Would you need multiple shapekeys or just the one?