As you know in 126.96.36.199 of the Guidelines and Standards for Tactile Graphic discusses the use of two column keys. I do not believe there is is an absolute right or wrong way to present the number of entries in each column. General thought is that the 2 columns should be divided as evenly as possible. You should also take into consideration the category of information contained in the two columns. If all the entries are alphabetic you could go down the first column in alphabetic order for the first 7 entries and then move to the second column and place the remaining 6 entries again in alphabetic order. Or you could put 6 in the first column and 7 in the last column so that the closing TN is definitely the last thing read in case the reader is reading from left to right instead of down. In reference to your original comments about having 9 lines of braille, my preference again would be to divide them as evenly as possible. Just because you have 9 lines of braille doesn't mean you have to use it.
I have also seen two column keys with different categories of entries, e.g. the first column had 3 line textures with definitions and the second column had 4 point symbols with definitions. The important thing is to place the closing TN after the last entry in the second column if that is indeed the last item in the key. Hope this helps!