Six is written as a bar and a dot. Beyond the notation, the sixth day of a trecena has no special standing, and this is worth saying plainly, because popular treatments of the count often assign a distinct character to each of the thirteen numbers as though the tradition supplied one.
What the sources actually record is thinner and more interesting. The Classic inscriptions give the coefficient as a numeral and nothing else; the codices use numbered days as coordinates in almanacs and tables; and living highland practice reads a day principally through its sign, with the number modifying rather than defining it, higher numbers generally understood as stronger. That is a graded scale, not thirteen separate characters.
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.
Rhythmic, the modern name for this coefficient, comes from Jose Arguelles's Dreamspell of 1987, a system that assigns each of the thirteen a fixed quality and a name. That assignment is the Dreamspell's own and should not be read back into the Maya count.
Sources Maya Hieroglyphic Writing: An IntroductionJ. Eric S. Thompson with Dreamspell: The Journey of Timeship Earth 2013Jose Arguelles