I once asked the same question of a truck body manufacturer's president of sales and the company's regional sales manager. They did not have a spec upon which they relied.
Not liking their answer and remaining curious, I started looking under every straight truck I saw. The number of U-bolts varies quite a bit from truck to truck (body manufacturer to body manufacturer). I only saw one truck where things were comming apart for want of U-bolts. That was on a brand-new truck where the lift-axle installer removed but did not replace some of them.
Otherwise, every U-bolt I have looked at has held firm. Four per side are more than some truck have. If you keep them torqued, I think you will do just fine. If you would feel better with more, it would be a simple matter to have a trailer shop add more.