It depends on the size of the holes or gaps. It’s true that mice only need about 6mm to get through. But what most people don’t know is that this needs to be a gap, not a hole. That’s because mice have to squeeze their bodies down to get through a small gap, and so their bodies go wide from the sides in the process. So if your air brick has only small holes less than 1 cm in width and height, it’s not likely that even the smallest mice would be able to get through. But if the airbrick is damaged, or otherwise has wide gaps, then mice might get through.