The saturation current depends on the intensity of light, not on frequency. Hence no matter how much is the frequency for the same intensity of light, the saturation photocurrent will be same.

Does photocurrent depend on frequency?

Current is not dependent on the frequency of photons. Frequency of photons is only related to the kinetic energy.

Why does photocurrent become saturated?

Now, in the curve between applied potential and photocurrent measured, on increasing potential, number of electrons reaching cathode per second increases. So naturally, the photocurrent also increases. But apparently, when "all" the electrons emitted are able to reach the cathode, the current reaches a saturation.