Albert Barnes Commentary Numbers 7:3

"and they brought their oblation before Jehovah, six covered wagons, and twelve oxen; a wagon for every two of the princes, and for each one an ox: and they presented them before the tabernacle." — Numbers 7:3 (ASV)

Covered wagons - Some prefer to render “litter (Isaiah 66:20) wagons:” that is, litters which were not on wheels, but borne by two oxen, one in front and one behind. Such conveyances would probably be more convenient than wheeled wagons in the rough country to be traversed.