To create a list variable, all you have to do is use a DITA2Go macro variable name with an index value in brackets, similar to C-language array notation:
$$listname[index]
$$mylist[$$_count] a variable as the index $$mylist[2] a constant as the index $$mylist[($$myindex + 1)] an expression as the index
The index is a string, not just a number, so it can be anything, even another nested list reference:
<$$mylist[$$another[one]]>
You can access the number of items in the list with <$$mylist[]>.