How many orders purchased by Aaron Bergman have been delivered with the slowest shipping speed?
slowest shipping speed refers to "Ship Mode" = 'Standard Class'
SELECT COUNT(*) FROM people AS T1 INNER JOIN central_superstore AS T2 ON T1.`Customer ID` = T2.`Customer ID` WHERE T1.`Customer Name` = 'Aaron Bergman' AND T2.`Ship Mode` = 'Standard Class'