As the penetration of wind power in power systems continues to rise, the inertia and frequency regulation capability provided by synchronous machines are gradually diminishing. This makes frequency support from wind power a necessary measure to ensure the secure operation of power systems. However, due to the inherent variability of wind power output, its frequency support capacity is time-varying. In addition, frequency support requirements differ under various system operating conditions, making it challenging to allocate frequency support tasks for wind turbines with different support capabilities under different scenarios. This paper proposes an optimal allocation strategy for frequency support requirements based on the evaluation of the available rotor kinetic energy of wind power. In scenarios where wind turbines release stored rotor kinetic energy to provide frequency support, an analytical expression for wind energy loss during frequency support is derived first based on the wind power capture characteristics, allowing accurate quantification of the available rotor kinetic energy. Subsequently, considering the actual power system operation conditions and system frequency security criteria, a method is proposed to assess the overall frequency support requirement for wind power. Finally, with the goal of minimizing wind energy loss, an optimal allocation strategy for frequency support demand among wind turbines is proposed. Simulation results show that the proposed method significantly reduces wind energy loss during frequency support while ensuring system frequency security.