Pilot sequences are the unmodulated data we are transmitting along with the data.Pilots are used for syncronization and Channel estimation purposes. Channel estimation is the main thing we need to concentrate for improving the channel capacity .
More the number of pilots more efficeintly we can estimate the channel and increase the capacity . But as pilots increases overhead increases. so the selection of pilots is such that tradeoff should be considered between pilots symbols and spectral efficiency. In IEEE 802.11 OFDM standard we are using 64 subcarreirs per OFDM symbol in these 64 , 48 are for data and 4 are Pilot and remaining are for zeropadding .
Hope you may get benefited from this.