First of all- hundreds of crows are more than likely rooks. Crows are solitary- they are usually seen alone or in pairs. Rooks tend to live in large colonies.
Secondly- albinism or part albinism- is quite common in birds. Often they are seen as 'different' by others of the same species, particularly those living in large flocks, and shunned.
Another option- where do you live? If you are still sure they are crows then there is always the possibility of a carrion crow/hooded crow hybrid. The hoodie is a subspecies of the crow and takes over from the all black crow north of Glasgow. The hoodie has a lot of grey plumage - see
http://www.rspb.org.uk/wildlife/birdguide/name /h/hoodedcrow/
Where the two species overlap you can get interbreeding which gives various combinations of grey and black plumage