how many students have a food allergy?
SELECT count ( * )  FROM Has_allergy AS T1 JOIN Allergy_type AS T2 ON T1.allergy   =   T2.allergy WHERE T2.allergytype   =   "food"
counting students with a specific characteristic refers to COUNT(*); food allergy is identified by allergytype = 'food'; joining tables to relate students and their specific allergies.