You need to go the FMCSA regs, Part 393, Subpart I (scroll down a little on the link and you'll come to it) and read and understand them throughly. 393.100-393.114 deal with what is acceptable for tiedowns and anchors. .128 deals with vehicled under 10,000# and .130 with those 10k or over.
Four is always better than two. And it can also depend on what you're using for restraints. In the under 10k class, let's assume you have a 9000# tractor. Your restraints on each end have to have a working load rating of at least half the weight of the tractor, so 4500 on each end.
Look in 393.108. There you'll find that a single G70, 5/16" (W[orking] L[oad] L[imit] 4700) or larger chain on each end will be legal. It's important that the chain be marked with its grade, as unmarked chain is assumed to be G30, in which case it would have to be 1/2" (WLL 4500)or larger to pass.
Scroll down just a little more and look at the straps. A common 2" strap is rated for 2000#, so you would need three on each end. A 3" strap is rated for 3k, so you could use two on each end. If the strap is tagged by the manufacturer for a greater WLL than the minimums I just cited, the rating on the tag will be used as long as the strap is in good condition, not frayed, UV rot . . .
BUT those are just minimums, and assume a straight line pull on the chains or straps. A little geometry and physics kick in, and you lose some of the restraining force once those restraints are pulled at an angle, as they typically are, so you don't want to have goods rated for bare minimum holding at an angle.
If the machine is 10k or over, you're obliged to tie down all four corners with restraints whose WLLs add up to at least half the weight. Also note that anything like a FEL, backhoe, anything mounted on a 3-point . . . . requires a separate restraint to hold it in place.
As for an enclosed trailer, see 393.102(c)(2). Two key points there. Adequate strength is one. It would take an extraordinarily strong structure (walls, headboard, doors, hinges and latches) to contain an unrestrained 9000# tractor. Also note the requirement for it being sufficiently close to the enclosure that it can't move. All this applies to loads on pallets or other containers that are sized to fill the width of a trailer, and are on friction mats or shored with bars or straps to prevent thim shifting for and aft. Which is all by way of saying that (c)(2) is not a gimme. (c)(1) and (c)(3), essentially the same as for an open trailer, will apply with a tractor.
All these ratings apply to EVERY component -- chains, straps, binders, tensioners AND anchor points.
Tie it down right for your own safety and that of others. If you do get caught, you'll find the LEO very unsympathetic, in fact, he'll likely red-tag you, if he has the sense that you were trying to hide a bad load in an enclosed trailer.
Part 393