Web-services-based parallel systems (WSB-P) should exhibit predictable behavior by guaranteeing quality-of-service parameters. One of the important requirements of predictability is that requests (tasks) should have maximum acceptable response time thresholds, which we denote as deadlines in this paper. In order to provide QoS, a WSB-P should try to guarantee these deadlines. Such policy affects the way the system should be designed, because all task durations must be under the deadline, and also how tasks are distributed among the servers. This way, it is desirable to use a load balancing algorithm that tries to both maximize the throughput and minimize the missed deadlines. This paper shows, with the help of a simulator, how the number of concurrent executions (CE) in the WSB-P plays a crucial role in the pursuit of the objective and how the optimal CE can be determined.