Simulation-based optimization

Hi, I am a begining user of Simulink. Here is my question: I built a simulation of process in a clinic,where there are 5 department (servers) here. The objective is to minimize the average of patients' total time in the clinic (maximize the total number of output in a certain simulation time). We have addtional fundations to add addtional staffs to each department (each server's capacity). Now I need to decide how to use this foundation to make a proper addtional staff assginment. The objective function is based on the simulation result. Could you tell me how to deal with this kind of problem?