Hello Dhanjay
I try to explain the points and question you raised about the parameters of OFDM
First of all, sequence length is the no. of bits transmit and the sequence length is directly relates to the word length that you can set from PRBS. In the case of OFDM the sequence length is long.
Samples per bit are the instants to sample the the one bit and total sampling freq. is your bit rate x sample per bits.Sample per bit in the case of OFDM are chosen to be less due to long sequence length which results into high no. of samples.
Gaurd bits are used to reduce ISI in the OFDM system.
Symbol rate quantifies the number of symbols transmitted per second.
No. of sample=Seq. length x Sample per bit