I move 8 all the time (with machines folded) in 6x12 trailers both U-Haul enclosed & my own 6x12 enclosed trailer. Doing 8 you can tie each machine body towards side wall and head towards center of trailer and 4 fit nicely down each side with about 3" to 4" between each head. I always toss big pieces of cardboard between each machine "just in case" but I have never had a machine move on me when well tied. Use good nylon rope to tie machines in the trailer (not something like cloths line). Rope will be a little expensive but it is still cheaper than damage to a couple machines. Ratcheting straps do not work well in U-haul trailers - nylon rope works out much better!

But if you are limited by towing capacity you might want to make two trips with 5 machines in each load. Regardless use an enclosed trailer and that way you don't have to worry about weather. If you do two trips with 5 in each load remember to center the machines over the axles instead of putting all the weight in the front of the trailer. One additional machine on one side is fine but when I do that I normally put the "extra" machine on drivers side of trailer since most roads are crowned towards shoulder. That way the trailer rides more level then putting additional machine on shoulder side.