The Flexible Manufacturing System (FMS) integrates humans, computers, robots, and computerized machines in order to meet system goals. The paper explores both the optimal allocation of tasks between human supervisor and computers and the optimal number of machines which a supervisor should control in an FMS. The hypotheses propose an interaction effect between the number of machines and the task allocation level and the existence of an optimal level of number of machines for the operator to monitor. These hypotheses were tested in a statistically balanced experimental design using 30 subjects with three independent variables: number of machines, task allocation, and sequence of task presentation. Results suggest that in the supervision of FMS, the task allocation to the supervisor is critical but that the size of the FMS system (4 to 20 machines), with the allocations studied, has no effect on either stress or performance of the operator in the FMS.