What is the wave number of beam of light in air, if its frequency is 14?10^14 Hz? (Velocity of light in air=3?10^8m/s).

Wave number is actually inverse of the wavelength 
you give            f = 14*10¹⁴Hz      
I think you know about              c = f  *  wavelenghth
so wavelength = c/f          
                        =  2.15 * 10^-7m
but wave number is inverse of wavelength 
   wave number = 4.6 * 10⁶ m^-1