I did it on 2-1 - others suggest 3-4 (theres a patch a the left with four sheeps on it). RNG put 5-6 sheep at the start. I waited until they got angry at me (the only way to influence their positioning directly), guided them to one place and once I had four close by, I whistled.

You have to make sure however that they have enough space to turn. So if they stand too close to each other it won't work.