To answer your question medhead. At first I thought you were just being facetious. But your answer is in the highlighted text of your OP.
You are voting for 12 senators and should vote 'at least' 12 below the line. But if you decide to number 1 to 64, then those preferences are counted. The solution is to number the voting paper up to the number you feel comfortable with, i.e. 1 to 12 or 1 to 25 or even 1 to 64. Not that difficult.
No votes extinguished or lost. Not quite sure why you want to go that method, but each to thier own.
So, in essence , there is no need to stop at #12.