When the channel bandwidth is selected at 28MHz(max), the max data rate depends on the modulation type being used in the OFDM subcarriers, such as BPSK, QPSK, 16QAM, 64QAM (2^N). The rough estimate for the max data rate will be channel BW x N.
Of course there are some guard and pilot sub-carriers that will slower actual data rate.
Also note, higher order modulation requires higher SNR.