Hi Duy Le Nguyen, maximun possible subcarriers and no. of fft points of new OFDM modulator are same.
I would like to suggest you not to use OS12 blocks, because it sometimes gives wrong result.
Did you mean out of 1024 possible subcarriers and 512 subcarriers were used?
If it is so, see the method of bandwidth calculation given in the component library that can be found in help. It is given as
OFDM symbol period = Symbol period * 128 = 128/(Symbol rate) = 2 * 128 /Global bit rate.
Are working on the sample files?
Regards.