How many American restaurants are located in Front, San Francisco?
American restaurant refers to food_type = 'american'; Front refers to street_name = 'front'; San Francisco refers to city = 'san francisco'
SELECT COUNT(T2.food_type = 'american') FROM location AS T1 INNER JOIN generalinfo AS T2 ON T1.id_restaurant = T2.id_restaurant WHERE T1.city = 'san francisco' AND T1.street_name = 'front'