Why we have to choose N=320 samples for distinguishing DTMF frequency…Can any body give me the exact proof?

Where did you get this figure from - sample counts will depend on sampling frequency of course. Is this an FFT based example you've been looking at?

Context is VERY important. Your question, as written, has none.

Links, example code, or a description will increase the likelyhood of an answer.

thejus08: Why we have to choose N=320 samples for distinguishing DTMF frequency.

Is this a homework question?