I use six if the car is going to be in the air more than a few hours - two per side on each side frame rail and one per side under the front of the frame to prevent frame creep. I, too, use wheels as an added measure of security under each frame rail whenever possible.
As has been said, don't scrimp on $$$$ when it's something that can very easily kill you. BUY AMERICAN - it costs more but I don't like the risks involved otherwise. I have some links at home for companies that sell US made ones and will post later. Lincoln used to sell all US made ones but their lower end units are now Chinese.