why the number of filter coefficients in FIR filter has to be an odd number? I calculated the order of the FIR filter to be 31(N). so, the number of coefficients has to be 32 (N+1). so, I have to increase it to 33 to make it an odd number.

Why the number of filter coefficients is required to be an odd number?
There is no such requirement. However, for linear-phase (symmetric) FIR filters, the delay can be expressed as:

$$
D = \frac{order}{2} = \frac{N_{taps} - 1}{2} \text{ samples}
$$

If the number of taps is odd, then the delay of the filter is an integer number of samples, which is desirable for some applications. If you have an even number of taps, then you end up with a half-sample delay.