In English, it's called "mode". I also have no idea how to code it, either. However, there are a few functions that might help:
dim(L1) (found in [2nd] (list), in the second submenu) returns the number of elements in the list. You can also store to this function to set the number of elements in a list.
L1(n) gets the nth element of the list, with 1 being the first element.