I put 4 on my 16' Scamp
, two crosswise about 1/2 way between the wheels and the rear, and two on the front Vee fore and aft. The different directions helps control the "wiggle" from side to side as they are pretty still along them.
I also use a 3/4" socket on a cordless drill and it works great. There is a tendency to jerk the drill and twist your wrist when the going gets stiffer.
I have used the crank to twist harder to level the trailer, raising the low side or end and then grounding the opposite to stabilize.
I have used this setup on the last trip and it is easy, quick and no need to get on my hands and knees to pull down Attwoods etc.
Much more stable as well.