Hello chandra sekhar,

X-rays are electromagnetic waves with wavelengths in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3×1016 Hz to 3×1019 Hz) and energies in the range 100 eV to 100 keV. They are shorter in wavelength than UV rays and longer than gamma rays.

Hello aspirant,

The order of em rays according to frequency is - 

Radio<Microwave<Infrared<UV<X-rays<Gamma

So, Gamma rays have highest frequency among all electromagnetic radiations. They are as radiations having frequency above X-rays, so there is no upper frequency limit for them. They can frequency as over as 10^19 Hz.

Hope this helps.