During one 12-hour shift, 800 non-defective parts are desired from a fabrication operation. The standard time for the operation is 18 minutes. Because the machine operations are unskilled, the actual time it takes to operate is 20 minutes, and on average, 10% of the parts that begin fabrication are scrapped. 

Assuming that each of the machines used for this operation will not be available for two hours each shift, determine the number of machines required.