How many drivers from the USA participated in the 2008 Australian Grand Prix?
from the USA refers to nationality = 'American'
SELECT COUNT(*) FROM drivers AS T1 INNER JOIN results AS T2 ON T1.driverId = T2.driverId INNER JOIN races AS T3 ON T3.raceId = T2.raceId WHERE T3.name = 'Australian GrAND Prix' AND T1.nationality = 'American' AND T3.year = 2008