One is written as a single dot, the simplest of the thirteen coefficients, and it has a structural role none of the others share: a day numbered one is always the first day of a trecena. The 260-day count is divided by its own arithmetic into twenty trecenas of thirteen days each, and each trecena is named for the day sign that carries the number one at its head.
That gives a second layer of organisation on top of the day-by-day sequence. A day is identified by its number and sign together, but it also sits at a known distance into a named thirteen-day run, and daykeepers work with both. The trecena beginning 1 Imix, the trecena beginning 1 Ik, and so on through the twenty, exhaust the 260 days exactly, because twenty trecenas of thirteen days are 260 days.
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.
The name Magnetic attached to this coefficient in much popular writing is not Maya. It belongs to the Dreamspell system published by Jose Arguelles in 1987, which borrows the thirteen-by-twenty structure and runs a separate count under invented tone names. In the living Maya count the coefficient is the number itself, jun in K'iche', and nothing more.
Sources Maya Hieroglyphic Writing: An IntroductionJ. Eric S. Thompson with Dreamspell: The Journey of Timeship Earth 2013Jose Arguelles