SNQ Signal Quality is scaled for each modulation type such that 50% is the minimum signal-to-noise requirement.
For ATSC1, 100% SNQ is 30dB+ SNR, 50% is 16dB SNR.
For ATSC3, the HDHR scales the MER such that 50% is the near-minimum SNR required for the given modulation(s) used on the channel.

It should contain as many as necessary.
http://wiki.xmltv.org/index.php/XMLTVFormat