I use four scissor jacks, two on each side just behind and just in front of the wheels. Allows easy final leveling / fine adjustment.
I use two scissor jacks at the front corners (but wish they were just A-arm types)
I use two A-arm types at the back corners, where it is unsafe to put too much force on the frame, just to stabilize.
This summer I added one A-arm type (since I had a spare in my garage) right at the steps, that made a HUGE difference in the stability of the trailer. No more shaking when people come in and out of the trailer. I'm only 150 lbs and I'm the heaviest member of the family, but there was still a lot of movement in the trailer when I came in and out on those steps.