Among the coaches who are born in the USA, how many of them used to train the Philadelphia Flyers?
born in the USA refers to birthCountry = 'USA'; Philadelphia Flyers is the name of team;
SELECT COUNT(DISTINCT T3.coachID) FROM Coaches AS T1 INNER JOIN Teams AS T2 ON T1.year = T2.year AND T1.tmID = T2.tmID INNER JOIN Master AS T3 ON T1.coachID = T3.coachID WHERE T2.name = 'Philadelphia Flyers' AND T3.birthCountry = 'USA'