How many students have milk or egg allergies?
SELECT count ( * )  FROM has_allergy AS T1 JOIN Student AS T2 ON T1.StuID   =   T2.StuID WHERE T1.allergy   =   "Milk" OR T1.allergy   =   "Eggs"
counting students = COUNT(*); milk or egg allergies refers to allergy = 'Milk' OR allergy = 'Eggs';