The least you can do it with is three but you MIGHT be able to get four. I did this;
One for holding first button, one for holding second, one for clicking box+half of one, three for three buttons, then last one for finish. I got the box to zero with one cursor but then with the next one I just sped it up. By the time it was done I had no time to get too the last three buttons, but if someone is even faster at clicking they might get four cursors left over.