Twelve is written as two bars and two dots, and it is the penultimate day of a trecena. There is no Maya cycle of twelve in the calendar, which is worth noting because a reader arriving from the European zodiac or the Chinese branches may expect one; the Maya periods are five, nine, thirteen, twenty, 260 and 365.
That difference in the basic numbers is a good guard against false equivalence. The twenty-eight Chinese xiu, the twenty-seven Indian nakshatras and the twenty-eight Arabic manazil are all lunar mansion systems and can be compared with care; the Tzolk'in is not a lunar system at all and has no counterpart outside Mesoamerica.
Maya numerals are written with a dot for one and a bar for five, and the coefficient of a day is written in exactly that way in the inscriptions, set beside the day sign in its cartouche. The system is positional and includes a sign for zero, which is why the Long Count can be written as a string of places, but within the 260-day count no number above thirteen ever occurs and the notation stays simple.
Crystal, the name attached to this coefficient in modern popular writing, comes from Jose Arguelles's Dreamspell of 1987. The atlas records the Dreamspell as a modern invention with a named author and a date, and records the Tzolk'in as an ancient count still kept, and does not allow either description to borrow the other's authority.
Sources Maya Hieroglyphic Writing: An IntroductionJ. Eric S. Thompson with Dreamspell: The Journey of Timeship Earth 2013Jose Arguelles