Hello Syed Hasan Asghar Naqvi,

you wrote:
i want to know that is there any maximum limit for the pin of 89c2051 of sensing high low inputs.

With a crystal frequency of 16 MHz the maximum rate a 89C2051 could count would be about 666 kHz.

This should be more than enough for you project. It sounds like you are counting using sensing when the signal is high and when the signal is low. If so, I would suggest you change it to an edge triggered method using one of the external interrupt.

Regards,

Charles Bannister