First thing, KOTA has a quota of 4 seats only. So for 20 tickets you'd have to book from NZM and change the boarding point to KOTA while booking.
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Second, for a date like December 28th, the seats will vanish within minutes of opening. So book it bang at 08:00 AM on October 29th.
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If...
more... you wish to go for group booking to ensure that you get seats together, its a tedious process. Refer to
/faq/post/527