Blood for transfusion needs to be crossmatched against antigen types of the recipient. Many patients will tolerate several transfusions of a minor mismatched antigen before developing a sensitivity. Major antigens are what cause significant reactions that can be life-threatening.

Minor antigens come into play when crossmatching for infants and premies, but this is way beyond my scope.

There are effectively millions of types because all the systems combined combinatorially, but most antigens beyond ABO and Rh don't cause that much of an issue, so in emergency cases, they just go with them.