Hi Anil !
The Max freq U can generate with 12MHz clock is 500KHz (Theoretical )..and I guess Practically some where around 450 KHz.
assuming that U'r Microcontroller divides clock by 12 as any normal 8051. so effective internal clock would be 1MMHz, and to get a cycle , U'll need a High State and a Low state .. Assuming you just output a High(+5v) followed by Low (0v).. with no delay in between and not using counter just using any port, ..then it should be around 500KHz.
With 50% Dutycycle, total time is (for 360 deg) = 0.002x10^-3 Sec
so Half of it (180Deg)... = 0.001x10^-3Sec (a High State)
...and the total time for 1MHz (180Deg) = 0.001x10^-3Sec (1/f)
Regards
Vignesh baliga