Wireless power transfer (WPT) technology is becoming popular and widely used in various fields including smart phones and electrical vehicles (EV). Therefore, designing an efficient and energy-saving WPT system is a critical issue. The focus of this paper is on a resonant coupled WPT system design by considering the scenario that more than one receivers present at the receiver side. The WPT system is optimized by minimizing the transmitted power from the transmitter side while considering the minimum required power at all receivers simultaneously. Using the tools of optimization theory, an optimization problem is formulated first that captures the multi-receiver WPT optimal design, and then it is reformulated as an equivalent convex program which is convenient to address. The paper also analyzes the impact of design parameters on the power transfer characteristics. Finally, a number of case studies are used to corroborate theoretical findings and provide optimal design guidelines for the resonant coupled multi-receiver WPT systems.