Among all the orders made by Aimee Bixby, how many of them chose the slowest delivery speed?
made by Aimee Bixby refers to "Customer Name" = 'Aimee Bixby'; the slowest delivery speed refers to "Ship Mode" = 'Standard Class';
SELECT COUNT(DISTINCT T2.`Order ID`) FROM people AS T1 INNER JOIN central_superstore AS T2 ON T1.`Customer ID` = T2.`Customer ID` WHERE T1.`Customer Name` = 'Aimee Bixby' AND T2.`Ship Mode` = 'Standard Class'